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Embryo-maternal crosstalk is essential to establish pregnancy, with the equine embryo moving throughout the uterus on days 9-15 (ovulation = day 0) as part of this interaction. We hypothesized that the presence of a mobile embryo induces local changes in the gene expression of the endometrium. On Day 12, the endometrial transcripts were compared among three groups: uterine horn with an embryo (P+, n = 7), without an embryo (P-, n = 7) in pregnant mares, and both uterine horns of nonbred mares (NB, n = 6). We identified 1,101 differentially expressed genes (DEGs) between P+ vs. NB and 1,229 DEGs between P- vs. NB. The genes upregulated in both P+ and P- relative to NB were involved in growth factor pathway and fatty acid activation, while downregulated genes were associated with oxytocin signaling pathway and estrogen receptor signaling. Comparing the transcriptome of P+ to that of P-, we found 59 DEGs, of which 30 genes had a higher expression in P+. These genes are associated with regulating vascular growth factors and the immune system, all known to be essential in early pregnancy. Overall, this study suggests that the mobile embryo influences the endometrial gene expression locally., (© 2024. The Author(s).)

Vitrified-warmed immature equine oocytes are able to complete the first meiotic division, but their subsequent developmental competence is compromised. Therefore, the present study investigated the effects of vitrifying immature horse oocytes on the chromosome and spindle configuration after IVM. Cumulus-oocytes complexes (COCs) were collected and divided into two groups based on mare age (young ≤14 years; old ≥16 years). COCs were then either directly matured invitro or vitrified and warmed before IVM. Spindle morphology and chromosome alignment within MII stage oocytes were assessed using immunofluorescent staining, confocal microscopy and three-dimensional image analysis. Vitrification reduced the ability of oocytes to reach MII and resulted in ultrastructural changes to the meiotic spindle, including shortening of its long axis, and an increased incidence of chromosomes failing to align properly at the metaphase plate. We hypothesise that aberrant chromosome alignment is an important contributor to the reduced developmental competence of vitrified equine oocytes. Contrary to expectation, oocytes from young mares were more severely affected than oocytes from older mares; we propose that the reduced effect of vitrification on oocytes from older mares is related to pre-existing compromise of spindle assembly checkpoint control mechanisms in these mares.

Invitro embryo production is an increasingly popular means of breeding horses. However, success is limited by a high incidence of early embryo loss. Although there are various possible causes of pregnancy failure, chromosomal abnormalities, including aneuploidy, are important potential contributors. This study evaluated the frequency of micronucleus formation as a proxy for aneuploidy in invitro-produced (IVP) and invivo-derived horse blastocysts. Associations between IVP embryo morphology, frequency of nuclear abnormalities and the likelihood of pregnancy were investigated. IVP blastocysts exhibited a higher frequency of cells with micronuclei than invivo-derived embryos (10% vs 1% respectively; P=0.05). This indication of chromosomal instability may explain the higher incidence of pregnancy failure after transfer of IVP embryos. However, the frequency of micronuclei was not correlated with brightfield microscopic morphological characteristics. Nevertheless, IVP embryos reaching the blastocyst stage after Day 9 of invitro culture were less likely to yield a pregnancy than embryos that developed to blastocysts before Day 9 (27% vs 69%), and embryos that had expanded before transfer were more likely to undergo embryonic death than those that had not expanded (44% vs 10%). These findings indicate that current embryo culture conditions are suboptimal and that the speed of embryo development is correlated with pregnancy survival.

The aim of this study was to evaluate the viability (percentage of dead cells) and the incidence of DNA fragmentation of horse embryos after storage in three different media at 5 degrees C for 6 and 24 h. Forty embryos were stored in Emcare Holding Solution for 6 and 24 h, in Hams'F10 or Vigro Holding Plus for 24 h at 5 degrees C (n = 9-10 per group) and 10 embryos were evaluated immediately after collection. First, embryos were stained, immediately after collection or following storage, to detect dead cells (DAPI) and, subsequently, DAPI-stained embryos were fixed and stained to detect DNA fragmentation (TUNEL). Finally, all the fixed embryos were re-stained with DAPI to determine the total number of cells. The percentage of cells stained with both TUNEL and DAPI or TUNEL-only or DAPI-only were determined. The percent of dead cells (DAPI-labelled) per embryo increased with duration of storage, but no differences were detected between the storage media. The percentage of early apoptotic cells (TUNEL+/DAPI-) in fresh and stored embryo for 6 h or 24 h did not differ significantly (P > 0.05). There was a significant correlation between the percentage of cells labelled by TUNEL and DAPI (R = 0.87) (P < 0.001). These results suggest that cooled storage increases cell death but this does not appear to occur by induction of apoptosis and that DAPI staining proves to be a quick and reliable method for assessing embryo viability.

Luteal steroids are necessary to maintain the first 70-90 days of pregnancy in the mare. At 35 days postovulation, the resurgence of the primary corpus luteum (CL) coincides with the secretion of the fetal hormone eCG. In order to study the responsiveness of the primary CL to eCG, we have examined levels of luteal equine LH/CG receptors (eLH/CG-R) mRNAs by Northern blot analysis and measured concentrations of eLH/CG binding sites on luteal membranes using 125I-eLH saturation binding assays at three stages of gestation: before the onset of eCG secretion (Days 14-31), from onset to maximum eCG secretion (Days 38-62), and during decline of eCG secretion (Days 83-101). Multiple transcripts of eLH/CG-R (7, 5.7, 4.9, 3.9, 2.8, 1.8, 0.6 kilobase [kb]) were identified in the primary CL at all stages examined. Three of them (5.7, 2.8, 0.6 kb) coded for truncated eLH/CG-R lacking the transmembrane domain. The relative intensities of the four major transcripts tended to decrease (5.7 and 3.9 kb) or were steadily expressed (7 and 1.8 kb) during pregnancy. The affinity of eLH/CG binding sites did not change during pregnancy whereas the number of eLH/CG binding sites decreased significantly after the onset of eCG secretion. Nevertheless, levels of binding sites were still at 44.6% (Days 38-62) to 24.7% (Days 83-101) of those measured before the onset of eCG secretion. Taken together, the presence of eLH/CG-R mRNAs and of a substantial part of eLH/CG binding sites with high affinity suggest that the primary CL still expresses a high number of eLH/CG-R and remains responsive to eCG during early pregnancy.

Equine embryos have been successfully transferred after 24h cooled storage in Ham's F-10. The aim of this study was to compare the viability of equine embryos in vitro and in vivo after 6 and 24h cooled storage using three media and to examine the relationship between embryo size and viability after 24h cooled storage. In Experiment 1, the viability of embryos was evaluated using DAPI-staining after 0, 6 or 24h in Ham's F-10, 24h in Emcare embryo holding solution (EHS) or 24h in ViGro holding plus (VHP) (n=10/group). The mean number of dead cells was similar for embryos stored in Ham's F-10, EHS and VHP for 24h. Larger Day 7 embryos appear to withstand 24h cold storage better than small Day 7 embryos. The embryo quality for 24h cold storage was negatively correlated with size. In Experiment 2, 40 embryos were stored (n=20/group) either in Ham's F-10 or in EHS then transferred as pairs in recipient mares. Fifteen of the 20 recipient mares (75%) were pregnant. Out of 17 surviving embryos, 9 embryos (53%) were stored in Ham's F-10 and 8 (47%) in EHS. These results suggest that EHS and VHP offer a good alternative to Ham's F-10 for 24h cooled storage of equine embryos and that larger embryos may have a better viability after 24h of cooled storage than smaller embryos.

An attempt was made to elicit maternal behavior in non-parturient Welsh pony mares through a combination of hormonal treatment and vaginal-cervical stimulation (VCS). Lactation was induced in 16 nonpregnant, non-parturient mares via a combination of estradiol, progesterone and a dopamine antagonist (sulpiride). During the adoption trials, each lactating mare was confined behind a padded bar and a newborn foal was held near her head. Eight of the mares received two 3-min periods of VCS when the foster foal was introduced. Following VCS, the foal was released and its interactions with the adoptive mare observed until the acceptance criterion was met (i.e. the mare accepted the foal at the udder with no signs of aggression). The remaining eight adoptive mares were treated in the same manner but did not receive VCS. All 16 non-parturient mares eventually accepted and nursed their adopted foal. However, acceptance latencies were significantly shorter for mares in the VCS condition than for those without VCS, and did not differ between the VCS condition and a group of control mares with their biological offspring. In subsequent choice tests, both groups of foster mares (with/without VCS), like the control mares, displayed a preference for their 'own' foal. Once the non-parturient mares accepted their foster foal, their maternal behavior resembled that of control mothers. The positive effect of VCS on maternal acceptance may reflect a release of oxytocin triggered by this treatment., (Copyright 2002 Elsevier Science Inc.)

In the pregnant mare, luteal estrogen production increases at the onset of equine chorionic gonadotropin (eCG) secretion by endometrial cups. In previous studies, we have demonstrated that eCG stimulates luteal androgen and estrogen production in pregnant mares. To further elucidate the regulation of steroidogenesis within the equine corpus luteum (CL) of pregnancy, we examined the expression of 3beta-hydroxysteroid dehydrogenase (3beta-HSD), cytochrome P450 17alpha-hydroxylase/17,20 lyase (P450(17alpha)) and cytochrome P450 aromatase (P450(arom)) in luteal tissue samples collected during diestrus (Days 7 to 10) and pregnancy before (Days 29 to 35) and after (Days 42 to 45) the onset of eCG secretion. Immunoblot analyses revealed a single protein per enzyme with molecular weights of 48 kDa (3beta-HSD), 58 kDa (P450(17alpha)) and 56 kDa (P450(arom)). Steady-state levels of 3beta-HSD were lower in luteal tissue of diestrus than pregnancy, but expression did not change during pregnancy. Steady-state expression of P450(17alpha) in CL of diestrus was not significantly different from that of pregnancy. During pregnancy, P450(17alpha) expression was significantly higher after the onset of eCG secretion. Steady-state expression of P450(arom) in CL of diestrus was not significantly different from that of pregnancy. During pregnancy, luteal expression of P450(arom) was significantly lower after the onset of eCG secretion. These data support the hypotheses that eCG has a differential effect on the expression of luteal steroidogenic enzymes, that the eCG-induced increase in luteal estrogen production is the result of an increase in available aromatizable androgen due to an increase in P450(17alpha) expression and activity, and that increased luteal estrogen production is not due to an increase in aromatase expression.

The aim of this study was to characterize changes in PGF2alpha secretion in mares with persistent corpora lutea that were induced by administering altrenogest during oestrus. In Expt 1, PGF2alpha secretion was compared among mares undergoing normal oestrous cycles (n=7) and mares undergoing prolonged luteal phases (n=6), using the mean 15-ketodihydro-PGF2alpha (PGFM) plasma concentrations, peak PGFM concentrations and number of PGFM surges each day, from day 12 to day 16 of the luteal phase. In Expt 2, oxytocin-induced PGF2alpha secretion was characterized on days 13 and 16 of the luteal phase in mares undergoing normal oestrous cycles (n=6) and in mares undergoing prolonged luteal phases (n=7) by comparing the oxytocin-induced increase in PGFM concentration and total PGF2alpha secretion. In Expt 1, mean PGFM concentrations, peak PGFM concentrations and number of PGFM surges per day were significantly lower in mares undergoing prolonged luteal phases than in mares undergoing normal luteal phases. In Expt 2, the area under the curve for PGFM ng (90 min)(-1) was similar for both groups on day 13 but was significantly lower on day 16 in mares undergoing prolonged luteal phases than in mares undergoing normal luteal phases. No change in total PGF2alpha secretion was observed between day 13 and 16 for mares undergoing normal luteal phases, but a significant decrease was observed from day 13 to day 16 in mares undergoing prolonged luteal phases. On days 13 and 16, the increase in PGFM concentration 5 min after oxytocin administration was significantly higher in mares undergoing normal luteal phases than in mares undergoing prolonged luteal phases. The increase in PGFM concentration 5 min after oxytocin administration was similar on days 13 and 16 for mares undergoing normal luteal phases, but tended to be less on day 16 in mares undergoing prolonged luteal phases. These results indicate that failure of luteolysis in mares undergoing induced prolonged luteal phases is due to decreased uterine sensitivity to oxytocin stimulation or decreased uterine ability to secrete prostaglandin.

At the onset of equine chorionic gonadotrophin (eCG) secretion, eCG stimulates luteal androgen and oestrogen production. Although eCG concentrations increase exponentially from day 37 to day 60 of gestation and eCG is detectable in maternal serum until about day 120-150 of gestation, luteal androgen and oestrogen production peaks between 5 and 10 days after initial exposure to eCG and then decreases gradually. It is not clear how eCG regulates luteal androgen and oestrogen production. In the present study, the steady-state mRNA expression of 3beta-hydroxysteroid dehydrogenase (3beta-HSD), cytochrome P450 17alpha-hydroxylase/17,20-lyase (P450(17alpha)) and cytochrome P450 aromatase (P450arom) in primary corpora lutea before, during and after eCG secretion was determined by northern blotting. Expression of 3beta-HSD was similar at all the stages examined. Cytochrome P450(17alpha) expression increased at the onset of eCG secretion, decreased between days 42 and 46 of gestation and was constant for the remaining period of eCG secretion. Cytochrome P450arom expression was highest before and after eCG secretion and lowest during periods of peak eCG secretion. The differential expression of P45017alpha and P450arom indicates that production of luteal androgen and oestrogen is regulated by P450(17alpha), activity. The effect of eCG on luteal steroidogenic enzyme mRNA expression appears to be stage-specific.

The effect of the dopamine antagonist sulpiride on FSH secretion and onset of reproductive activity in anoestrous mares under different environmental conditions was investigated. In Expt 1, sulpiride (0.5 mg (-)-sulpiride kg(-1) twice a day) had no affect on FSH pulse frequency, mean FSH concentration, basal FSH concentration or FSH pulse amplitude in anoestrous mares. These data do not support the hypothesis that dopamine inhibits reproductive activity by suppressing GnRH secretion, as it does in other species. In Expt 2, the interval to first ovulation (14.8 +/- 1.9 days; range 12-22 days) in five mares treated with sulpiride (0.5 mg (-)-sulpiride kg(-1) twice a day) housed indoors under extended daylength (16 h light: 8 h dark) was significantly shorter (P < 0.02) than in six untreated mares housed indoors under extended daylength (34.3 +/- 5.5; range 16-52 days and seven untreated mares housed outside under natural photoperiod (73 +/- 10; range 37-107 days). However, if the FSH secretion parameters at the start of treatment are treated as covariants, each has a significant effect (P < 0.05) on the interval to ovulation and sulpiride treatment does not have a significant effect. In Expt 3, the interval to first ovulation was not significantly different in sulpiride-treated (200 mg (-)-sulpiride twice a day) and untreated mares maintained outside under natural photoperiod. These results indicate that sulpiride treatment combined with increased temperature (indoor housing) and stimulatory photoperiod (extended daylength) results in a shorter interval to first ovulation and that a nonstimulatory environment decreases the effect of treatment on the interval to first ovulation. The role of FSH secretion at the time of treatment remains to be determined.

The onset of eCG secretion in pregnant mares coincides with an increase in luteal steroid production and a relative shift toward androgen and estrogen synthesis. However, a cause-effect relationship between eCG and the shift in luteal steroidogenesis has not been demonstrated. In this study, we have investigated the effect of eCG on steroid production by the corpus luteum (CL) during equine pregnancy. All mares were supplemented with 44 mg altrenogest (a progestogen) per day on Days 18-50. Increasing doses of eCG were administered on Days 26-28, before the onset of endogenous eCG secretion, to four mares with and four mares without a functional CL (prostaglandin F2alpha administered on Day 18). Four mares with a functional CL received no exogenous eCG. In eCG-treated mares without a functional CL, progestin, androstenedione, and estrogen concentrations did not significantly increase after exogenous eCG administration or endogenous eCG secretion. In eCG-treated mares with a functional CL, progestin and estrogen production increased significantly after exogenous eCG administration and endogenous eCG secretion, whereas androstenedione concentrations tended to increase following exogenous eCG and increased significantly following endogenous eCG secretion. In mares with a functional CL that did not receive exogenous eCG, progestin and estrogen concentrations increased and androstenedione concentrations tended to increase only after the onset of endogenous eCG secretion. These data demonstrate that the increase in luteal steroidogenesis that coincides with the onset of eCG secretion is induced by eCG and results in an increase in luteal androgen and estrogen synthesis. Our findings support the hypothesis that eCG has a luteotropic action in pregnant mares.

Estradiol (E2), testosterone (T) and progesterone (P4) concentrations were determined by enzyme-immunoassay in aqueous extracts of fecal samples obtained during anestrus, proestrus, estrus and metestrus of 11 nonpregnant and 11 pregnant bitches. Fecal hormone concentrations (ng/g) changed in relation to stage of cycle. Mean fecal steroid concentrations in 22 anestrous bitches and 3 ovariectomized bitches were low and similar for E2 (53 +/- 5 and 27 +/- 2), T (60 +/- 7 and 36 +/- 6), and P4 (62 +/- 6 and 86 +/- 15). Within 0 to 3 d of the ovulatory LH surge fecal E2 reached peak concentrations (301 +/- 38). The T peaks (281 +/- 41) were coincident or 1 to 3 d later. Fecal P4 was then elevated for approximately 2 m.o. Between Days 26 and 45 after ovulation, mean fecal P4 concentrations were higher (P < 0.05) in pregnant (401 +/- 60) than in nonpregnant bitches (164 +/- 23) and peak fecal P4 concentrations in individual animals were higher (P < 0.01) in pregnant (812 +/- 121) than in nonpregnant bitches (425 +/- 97). In the same period mean concentrations of E2 (117 +/- 13 vs 61 +/- 5) and T (102 +/- 10 vs 70 +/- 6) were also higher (P < or = 0.05) in pregnant than in nonpregnant bitches. Serum E2, T and P4 concentration were positively correlated (P = 0.1) with concentration in fecal samples obtained one day after serum collection. Although serial fecal ovarian steroid concentrations demonstrate the time course of ovulatory cycles, the diagnostic value of individual fecal samples appears limited. The ratios of peak to basal values were approximately 6, 5 and 7 for E2, T and P4, respectively, and were considerably lower than ratios of 12 to 50 previously reported for serum or plasma concentrations. The results demonstrate that there are pregnancy-specific increases in P4, E2 and T production reflected in fecal concentrations. While such increases are reflected in fecal samples, they are generally not evident in serum or plasma concentrations because of increased hemodilution, metabolism and clearance in pregnant bitches. The physiological stimulus for these increases, presumably ovarian in origin, or the potential role of prolactin is not known.

The onset of equine chorionic gonadotrophin (eCG) secretion in pregnant mares is associated with an increase in luteal androgen and oestrogen production. The luteal cell type(s) responsible for the increased production of androgens and oestrogens has not been identified in the equine corpus luteum. In this study, we examined the pattern of expression of 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD), cytochrome P450 17 alpha-hydroxylase/17,20-lyase (P450(17 alpha)) and cytochrome P450 aromatase (P450arom) by immunohistochemistry in equine luteal tissue collected during dioestrus (days 7-10; n = 4) and early pregnancy, before (days 29-35; n = 4) and after (days 39-45; n = 4) the onset of endogenous eCG secretion. All luteal cells expressed 3 beta-HSD, P450(17 alpha) and P450arom. The distribution of 3 beta-HSD, P450(17 alpha) and P450arom did not differ with stage of the reproductive cycle. The intensity of immunohistochemical staining for 3 beta-HSD did not appear to differ with reproductive stage. In contrast, the intensity of immunostaining for P450(17 alpha) increased after the onset of eCG secretion. The intensity of immunostaining for P450arom increased during pregnancy before the onset of eCG secretion and diminished after the onset of eCG secretion to the intensity seen in dioestrous corpora lutea. This finding suggests that androgen and oestrogen production is not compartmentalized within the equine corpus luteum. Both large and small luteal cells express the steroidogenic enzymes necessary for oestrogen production, and the intensity of immunostaining for P450(17 alpha) and P450arom appears to be stage-specific.

In pregnant mares, eCG stimulates luteal androgen and estrogen production, increasing plasma concentrations 2- to 3-fold. To study how these changes are regulated, we examined the expression of mRNA for the steroidogenic enzymes 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD), cytochrome P450 17 alpha-hydroxylase/17,20-lyase (P450 17 alpha), and cytochrome P450 aromatase (P450arom) in equine primary corpora lutea using Northern blot analyses. Three equine specific cDNAs were generated by reverse transcriptase polymerase chain reaction. When compared to human, bovine, and rat sequences, the nucleotide identities were 82%, 84%, and 76%, respectively, for 3 beta-HSD cDNA (843 base pairs [bp]); 79%, 80% and 66% for P450(17) alpha cDNA (541 bp); and 80%, 83% and 75% for P450arom cDNA (289 bp). The P450(17) alpha cDNA sequence demonstrated 99.6% nucleotide identity with the previously published sequence for equine testicular P450(17) alpha. Luteal tissue samples were collected at three times: diestrus (Days 8-10), early pregnancy before the onset of eCG secretion (Days 29-35), and early pregnancy after the onset of eCG secretion (Days 42-45). Although no significant changes were observed in 3 beta-HSD expression, P450(17) alpha and P450arom demonstrated stage-specific transcriptional regulation. Steady-state levels of P450(17) alpha mRNA were similar during diestrus and early pregnancy before the onset of eCG secretion but increased significantly after the onset of eCG secretion. Cytochrome P450arom mRNA levels decreased significantly after the onset of eCG secretion. Steady-state levels of P450arom mRNA were highest in luteal tissue collected during pregnancy before the onset of eCG secretion and intermediate during diestrus. Secretion of eCG appears to increase luteal estrogen synthesis by a transcriptional up-regulation of P450(17) alpha expression. These data suggest that availability of aromatizable androgens may be rate-limiting in luteal estrogen synthesis before the onset of eCG secretion.

We investigated the role of dopamine in the regulation of seasonal reproductive activity in mares. Nine seasonal anestrous mares, maintained under a natural photoperiod, were treated daily with a dopamine D2 antagonist, [-]-sulpiride (200 mg/mare, im), beginning February 5 (day of year = 36) until the first ovulation of the year or for a maximum of 58. Nine untreated anestrous mares were maintained under the same conditions. The ovaries were examined by ultrasonography twice a week, and blood was collected three times a week for progesterone, LH, FSH and prolactin determinations. Mean day of first ovulation was significantly advanced for [-]-sulpiride-treated mares than control mares (mean day of year +/- SEM = 77.3 +/- 7.9 and 110.0 +/- 6.8, respectively; P < 0.01). Eight mares ovulated during [-]-sulpiride treatment while one mare failed to ovulate. Ovulation occurred 91 d after the start of treatment or on Day 127. All mares continued to have normal estrous cycles after the first ovulation. First cycle length and luteal progesterone concentrations did not differ between [-]-sulpiride-treated and control mares. Plasma prolactin concentrations were significantly increased at 2 and 9 h after [-]-sulpiride administration (P < 0.05), and had returned to basal levels by 24 h. At the time of the LH surge associated with the first ovulation, mean LH and FSH secretion was significantly higher in [-]-sulpiride-treated mares than in control mares (P < 0.05). These results suggest that dopamine plays a role in the control of reproductive seasonality in mares and exerts a tonic inhibition on reproductive activity during the anovulatory season.

Two experiments were conducted to test the efficacy of altrenogest treatment in mares. The response to 15-d altrenogest treatment (Experiment 1) was characterized in 20 mares that were given 22 mg daily of altrenogest in oil (n = 10) or in gel (n = 10) from Day 10 to 25 after ovulation. In 17 mares, luteolysis occurred during altrenogest treatment (Day 17.7 +/- 0.5), while 2 mares retained their corpus luteum (CL), and 1 mare had a diestrous ovulation on Day 16, resulting in a prolonged luteal phase. Ten of the 17 mares in which the CL had spontaneously regressed returned to estrus after the end of treatment, and ovulated 5.7 +/- 0.8 d after the end of altrenogest treatment. Two of these 17 mares ovulated 2 and 3 d after the end of altrenogest treatment but ovulation was not accompanied by estrous behavior, and 5 mares ovulated during altrenogest treatment resulting in an interovulatory interval of 22.4 +/- 1.1 d (range: 20 to 25d). Five mares which ovulated during altrenogest treatment and 2 mares which ovulated during silent estrus after the end of altrenogest treatment failed to regress the CL around 14 d post ovulation, and had a prolonged luteal phase. In Experiment 2, the effect of altrenogest administered from luteolysis to ovulation on duration of the subsequent luteal period was analyzed. In 6 mares altrenogest was begun on Day 14 post ovulation and continued until the hCG-induced ovulation. The interval from ovulation during altrenogest treatment to spontaneous luteolysis was 45.6 +/- 2.4 d (range: 40 to 54d) in altrenogest-treated mares and was significantly longer than in 10 untreated control mares (14.5 +/- 0.3 d, range: 13 to 16d). The results suggest that the oil and gel altrenogest preparations are equally effective in modulating estrous behavior and time to estrus and ovulation. Altrenogest treatment started late in diestrus appears to result in a high incidence of ovulation during treatment and when luteolysis and ovulation occur during treatment; the subsequent luteal phase is frequently prolonged due to failure of regression of the CL.

We have previously demonstrated that daily administration of the dopamine D2 antagonist, sulpiride, during seasonal anoestrus, effectively advances the mean time of onset of the breeding season in mares. The purpose of this study was to examine the effect of sulpiride administration on pulsatile FSH and LH secretion in seasonally anoestrous mares, follicular development, time of first ovulation and the fertility at the first ovulation. Fourteen anoestrous mares were selected based on progesterone concentrations < 1 ng ml-1 for 3 weeks and largest follicle diameter < 20 mm. Starting 30 January, eight seasonally anoestrous mares were treated daily with sulpiride until the first ovulation of the year, and six untreated control mares were maintained under the same environmental conditions. Ovarian activity was monitored and plasma samples were collected every other day. On days 1, 11 and 21 of treatment, plasma samples were collected every 15 min for 11 h in six treated and six control mares. Mares were bred during the first oestrus. Mean time of first ovulation was significantly advanced in sulpiride-treated mares compared with control mares. Pregnancy rate 18 days after ovulation was similar between groups. Mean FSH pulse frequency on the first day of treatment and mean plasma FSH concentrations on day 11 of treatment were significantly higher in sulpiride-treated mares compared with control mares. No significant difference was observed between groups for parameters of LH pulsatile secretion. The results of this study suggest that dopamine inhibits FSH pulsatile secretion in seasonally anoestrous mares.

Objectives: To determine the role of progesterone in the regulation of endogenous prostaglandin F2 alpha (PGF2 alpha) secretion during cloprostenol-induced abortion and to investigate use of progestins to prevent prostaglandin-associated abortion., Animals: 16 pregnant mares., Procedure: To induce abortion, cloprostenol (250 micrograms/d) was administered daily until fetal expulsion or for up to 5 days. In experiment 1, 8 mares, 98 to 153 days' pregnant, received progesterone (300 mg/d) at 24-hour intervals for 5 days, starting 18 hours after the first cloprostenol administration. In experiment 2, 8 mares, 93 to 115 days' pregnant, received altrenogest (44 mg/d) at 24-hour intervals, starting 12 hours after the first cloprostenol administration. Historic control mares, 82 to 102 days' pregnant, received cloprostenol (250 micrograms/d) daily until fetal expulsion., Results: In control mares, fetal expulsion occurred after 2 to 3 cloprostenol administrations and was associated with significant increases in PGF2 alpha secretion. Abortion did not occur in 5 of 8 progesterone-treated mares and 8 of 8 altrenogest-treated mares, and endogenous PGF2 alpha secretion was inhibited, compared with values in aborting mares., Conclusion: Circulating progestogen concentrations may have a role in the outcome of pregnancy during prostaglandin-induced abortion. Altered prostaglandin secretion may be a reflection of a direct effect of progesterone or may be caused by the abortion process., Clinical Relevance: Progestogens might be useful for prevention of abortion in mares in which pregnancy is at risk owing to diseases that are associated with excess prostaglandin secretion.

In the horse mare, the onset of parturition is associated with an increase in oxytocin secretion, and it has been suggested that the onset of parturition may be triggered by endogenous oxytocin release. To test the hypothesis that oxytocin secretion is regulated by endogenous opioids in the periparturient period, we have 1) characterized oxytocin secretion in response to vaginocervical stimulation and 2) determined the effect of naloxone, an opioid antagonist, on oxytocin secretion induced by vaginocervical stimulation in prepartum mares and in postpartum mares at estrus and diestrus. During the last 2 months of pregnancy, the first diestrus and subsequent estrus post partum, a total of 66 vaginocervical stimulations were performed. Mares were pretreated with naloxone (0.5 mg/kg i.v.) or saline, administered 20 min before vaginocervical stimulation on subsequent days, using a randomized switchback design in which mares served as their own controls. Plasma was collected from 30 min before until 30 min after stimulation and was analyzed for oxytocin concentrations. Vaginocervical stimulation resulted in a significant increase in oxytocin secretion in all mares. Between Days 30 and 20 prepartum, the total amount of oxytocin secreted (calculated as area under the curve for 0 to 10 min after vaginocervical stimulation) was significantly greater in naloxone-treated than in saline-treated mares. From Day 20 prepartum until parturition, the differences between naloxone and saline-treated mares tended to decrease with approaching parturition, and were no longer statistically different. Peak plasma oxytocin concentrations were greater in naloxone-treated mares than in saline-treated mares during the entire prepartum period. During the postpartum period, total amount of oxytocin secreted following vaginocervical stimulation tended to be greater than during the prepartum period, and stimulated oxytocin secretion was significantly greater in naloxone-treated mares than in saline-treated mares. In conclusion, these data suggest that endogenous opioids suppress oxytocin secretion pre and post partum. It appears that opioid inhibition is not limited to the prepartum period, tends to decrease gradually towards parturition and is reinstated after foaling.

We have characterized the testosterone secretion pattern during the first 80 d of pregnancy in mares and determined the sources that contribute to circulating testosterone levels during this period. Ten untreated, pregnant mares (Group 1), 10 altrenogest-treated, pregnant mares (Group 2), and 10 altrenogest-treated, pregnant mares in which the CL was eliminated by administration of PGF-2alpha on Day 16 (Group 3) were used in this study. Complete luteolysis occurred following PGF-2alpha administration in all mares in Group 3. Six of the 10 mares in Group 3 did not have an active CL until after Day 60 of pregnancy (Group 3a) and were included in the analysis. The remaining four mares developed a new CL on Days 32, 40, 43 and 49 of pregnancy and were excluded from analysis. Mares without a functional CL (Group 3a) had significantly lower testosterone concentrations than mares with a functional CL (Groups 1 and 2), during the period before equine chorionic gonadotropin (eCG) secretion. At the onset of eCG secretion, testosterone concentrations increased rapidly but the rate of increase decreased with time in mares with a functional CL (Groups 1 and 2). In mares without a functional CL (Group 3a), testosterone concentrations did not increase at the onset of eCG secretion but increased at a gradually increasing rate after Day 50. The lower testosterone concentration in mares without a functional CL before eCG secretion suggests that the CL contributes significantly to the circulating testosterone concentration during the period before eCG secretion. The close time relationship between the onset of eCG secretion and the increase in testosterone secretion in mares with a functional CL and the lack of a testosterone increase in pregnant mares without a functional CL suggest that the increase in testosterone secretion after Day 35 of pregnancy is the result of eCG-stimulated, luteal testosterone synthesis.

Objective: To determine the relative role of endogenous prostaglandin F2 alpha (PGF2 alpha) secretion in cloprostenol-induced abortion in mares that no longer require luteal progesterone secretion for maintenance of pregnancy, and to evaluate the ability of a prostaglandin cyclooxygenase inhibitor (flunixin meglumine) to prevent cloprostenol-induced abortion., Design: The effect of flunixin meglumine on PGF2 alpha secretion and outcome of pregnancy was compared between mares treated with cloprostenol only and mares treated with cloprostenol plus flunixin meglumine., Animals: Five pregnant mares, aged 4 to 15 years, of light-horse type., Procedure: Cloprostenol (250 micrograms) was administered at 24-hour intervals to 5 pregnant mares. Flunixin meglumine (500 mg, IV) was administered at 8-hour intervals starting 15 minutes before the first cloprostenol administration. Hourly blood samples were analyzed for 15-ketodihydro-PGF2 alpha, progesterone, and estrogen concentrations. Previously reported data on cloprostenol-induced abortion in 6 pregnant mares treated daily with cloprostenol only were used as historic controls., Results: The mean (+/- SEM) interval from first cloprostenol administration to fetal expulsion 56.4 (+/- 13.7) hours and number of cloprostenol administrations 3.2 (+/- 0.6) in the 5 flunixin meglumine-treated mares were not significantly different, compared with values for 6 pregnant mares treated daily with cloprostenol only, 48.6 (+/- 5.6) hours and 2.8 (+/- 0.2) cloprostenol administrations. Flunixin meglumine did not inhibit endogenous PGF2 alpha secretion. Prostaglandin F2 alpha secretion rates on the day before and day of fetal expulsion were similar in both groups., Conclusion: Flunixin meglumine at a dosage of 500 mg/animal, administered IV every 8 hours, is ineffective in modulating uterine PGF2 alpha secretion during cloprostenol-induced abortion., Clinical Relevance: Flunixin meglumine is ineffective in the modulation of prostaglandin-induced uterine PGF2 alpha secretion and, therefore, does not offer a viable alternative for the prevention of abortion in mares at risk of abortion because of systemic illness.

The aim of the present study was to investigate the role of ovarian steroids in the opioid regulation of LH and prolactin release in mares. Effects of the opioid antagonist naloxone on LH and prolactin secretion were determined in ovariectomized pony mares. The animals were pretreated with either progesterone (500 micrograms kg-1) or oestradiol benzoate (10 micrograms kg-1) for 8 days and subsequently with a combination of progesterone and oestradiol for an additional 8 days. Naloxone administration (0.5 mg kg-1 i.v.) resulted in a significant release of LH as well as prolactin in mares after pretreatment with either oestradiol benzoate or progesterone plus oestradiol benzoate (P < 0.05). No significant changes in LH and prolactin secretion were detected in progesterone-treated and non-steroid-treated ovariectomized mares. These results indicate that a prolonged oestrogen influence activates the opioid inhibition of LH and prolactin release in mares. In contrast to other species, progesterone alone does not activate a tonic opioid inhibition of LH and prolactin secretion, but modulates the effect of oestrogens. The opioid systems therefore seem to be regulated by a sequence of different steroid environments, as found during the oestrous cycle. The parallel increases in prolactin and LH secretion in mares may indicate a common regulatory pathway for these two hormones.

The purpose of this study was to develop a contraceptive method for feral horses. The feral horse population has increased significantly in recent years despite attempts to control numbers. As in most wild animal population control programs, contraceptive methods must be easy to apply, cause minimal disruption to the social structure and be fully reversible. In the present study, we tested the effectiveness of an intrauterine device (IUD) for fertility control in mares. Six mares were fitted with a silastic O-ring-shaped IUD on July 1 of Year 1. The IUD-treated mares were turned out with 12 nontreated mares and a fertile stallion in a large pasture until October 20 (112 d). None of the IUD-treated mares and all the nontreated mares became pregnant. The IUD-treated mares were maintained separately from the stallion during the winter. Following removal of the IUD on April 27 of Year 2, the mares were again introduced to the pasture with the stallion together with 6 nontreated mares. For the 6 mares previously treated with an IUD, the mean interval from introduction to the stallion to conception was 17.5 +/- 5 d or 1.3 cycles per pregnancy, and all mares produced a normal foal at term. Subsequently, 19 recorded mare breeding seasons resulted in 18 foals. Uterine cytology and histopathology indicate that the IUD causes mild chronic endometritis without permanent changes in the endometrium. We conclude that based on our observations, the O-ring-shaped IUD is an effective, safe and practical contraceptive method for mares.

The objective of this study was to test the hypothesis that PGF2alpha is associated with abortion and changes in plasma Zn, Cu, and Fe concentrations in cows and mares in their first trimester of pregnancy. Eleven pregnant cows were infused with endotoxin (n = 5) or endotoxin plus an inhibitor of cycloxygenase, flunixin meglumine (n = 6). Blood was collected over a 5-d period. Additionally, 4 mares were treated every 24 h with cloprostenol sodium and blood was collected hourly until abortion. Plasma Zn, Cu, and Fe were determined. Three of five cows treated with endotoxin aborted, but none of the six cows treated with endotoxin and flunixin meglumine aborted. Aborting cows had lower plasma Zn (P = 0.048) over the 5-d study period compared with the nonaborting cows. The changes in Zn corresponded to release of PGF2alpha. All 4 mares aborted and plasma Zn concentrations were lower (P = 0.008) and Cu/Zn was higher (P = 0.02) 12 h after cloprostenol treatment. Plasma Zn may be a useful biomarker for risk of spontaneous abortion, and the decline in plasma Zn may be caused by PGF2alpha.

The influence of exogenous progesterone on the development of equine oviductal embryos was determined based upon the recovery of Day-7 uterine blastocysts from treated mares (n=13) that were given 450 mg progesterone daily between Days 0 and 6 and from untreated control mares (n=13). Daily administration of 450 mg progesterone in oil significantly (P<0.02) increased serum progesterone concentrations in the treated mares. There was no significant difference in the recovery rate of Day-7 embryos between treated and control mares (8/13 versus 6/13, respectively). Embryonic development, assessed by morphologic evaluation, embryo diameter, and number of cell nuclei was not significantly different for embryos from treated and from control mares. The results of this study indicate that administration of progesterone beginning on the day of ovulation does not affect the embryo recovery rate or embryonic development, based on evaluation of uterine blastocysts recovered at Day 7 after ovulation.

Eighteen healthy, pregnant mares scheduled for laparotomy and uterine manipulation were randomly allotted to 2 equal groups. After IV administration of xylazine hydrochloride and thiamylal sodium, general anesthesia was maintained with halothane (HALO) or isoflurane (ISO) in oxygen. Results of cardiovascular measurements were similar with both inhalant anesthetics; mean arterial blood pressure was 79 and 82 mm of Hg with HALO and ISO, respectively. Respiratory rate decreased most with ISO (mean frequency was 4 and 9 breaths/min with ISO and HALO, respectively). Partial pressure of arterial CO2 was increased similarly with HALO and ISO. Partial pressure of arterial O2 varied greatly among mares and decreased with duration of use of both anesthetics. Recovery time from anesthesia was significantly (P < 0.05) shorter after use of ISO vs HALO. Minor superficial injuries were associated with recovery from both anesthetics (in 5 mares with ISO and in 1 mare with HALO). Physical signs of postanesthetic myopathy or vital-organ dysfunction were not associated with either agent.

Administration of progesterone in poly(d-,l-lactide) microspheres was used to maintain pregnancy in mares after luteolysis was induced by treatment with prostaglandin F2 alpha at day 14 of pregnancy. Mares were given vehicle only (control, n = 6) or 0.75 g (n = 7), 1.5 g (n = 8), or 2.25 g (n = 5) of microencapsulated progesterone at days 12 and 22 of pregnancy. Serum progesterone concentrations were determined daily, and pregnancy was evaluated by transrectal ultrasonography on alternate days. Significantly (P less than 0.05) more mares given 1.5 or 2.25 g of progesterone (6 of 8 and 4 of 5 mares, respectively), but not those given 0.75 g (3 of 7 mares), maintained pregnancy through day 32, compared with control mares (0 of 6). Progesterone concentrations decreased significantly (P less than 0.025) in all groups after administration of prostaglandin F2 alpha at day 14, and significant (P less than 0.05) effects of time and treatment on progesterone concentrations were found between days 12 and 22, and 22 and 32. Although treatment with 1.5-g and 2.25-g doses of microencapsulated progesterone improved maintenance of pregnancy, compared with that of vehicle-treated controls, administration of 2.25 g of microencapsulated progesterone appeared to be most efficacious in maintenance of pregnancy during the study interval.

A direct radioimmunoassay for estrogen conjugates (EC) was applied to paired blood and urine samples collected from 20 mares and compared against estrone (E(1)) and estradiol-17beta (E(2)) to monitor changes in estrogen production during ovulatory cycles and early pregnancy. Blood samples were taken daily from five mares through two consecutive ovulations and from six mares at 6-h intervals starting 48 hours prior to ovulation and continuing after ovulation had occurred. Blood samples were also collected daily or three times per week from conception until Day 60 of pregnancy in nine pregnant mares. The mean urinary EC, plasma EC and plasma E(2) dynamics were parallel in nonpregnant mares, with a 3-fold increase in mean urinary EC concentrations from baseline to the ovulatory peak, a 1.8-fold increase in mean plasma EC concentrations and a 1.4-fold increase in mean plasma E(2) concentrations. In early pregnancy, a two-fold increase in mean plasma E(1) and EC concentrations occurred in concert with a five-fold rise in mean urinary EC concentrations, whereas plasma E(2) did not change. Following hydrolysis and chromatographic separation, E(1) and E(2) were identified as the hydrolytic products in the urine of nonpregnant and pregnant mares; however, an unidentified estrogen was the major hydrolytic product in nonpregnant mares and pregnant mares prior to Day 38 of pregnancy. The increased resolution of the EC profiles compared with the profiles of other estrogen components indicates that the determination of EC in urine or plasma provides a useful alternative method for monitoring reproductive events in mares.

The role of decreased luteal activity in embryonic loss after induced endotoxemia was studied in mares 21 to 35 days pregnant. Fourteen pregnant mares were treated daily with 44 mg of altrenogest to compensate for the loss of endogenous progesterone secretion caused by prostaglandin F2 alpha (PGF2 alpha) synthesis and release following intravenous administration of Salmonella typhimurium endotoxin. Altrenogest was administered daily from the day of endotoxin injection until day 40 of gestation (group 1; n = 7), until day 70 (group 2; n = 5), or until day 50 (group 3; n = 2). In all mares, secretion of PGF2 alpha, as determined by the plasma 15-keto-13,14-dihydro-PGF2 alpha concentrations, followed a biphasic pattern, with an initial peak at 30 minutes followed by a second, larger peak at 105 minutes after endotoxin injection. Plasma progesterone concentrations decreased in all mares to values less than 1 ng/ml within 24 hours after endotoxin injection. In group 1, progesterone concentrations for all mares were less than 1 ng/ml until the final day of altrenogest treatment. In 6 of 7 mares in group 1, the fetuses died within 4 days after the end of treatment, with progesterone concentrations less than 1 ng/ml at that time. In the mare that remained pregnant after the end of treatment, plasma progesterone concentration was 1.6 ng/ml on day 41 and increased to 4.4 ng/ml on day 44. In group 2, all mares remained pregnant, even though plasma progesterone concentrations were less than 1 ng/ml in 4 of 5 mares from the day after endotoxin injection until after the end of altrenogest treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of prostaglandin F2 alpha (PGF2 alpha) in embryonic loss following induced endotoxemia was studied in mares that were 21 to 44 days pregnant. Thirteen pregnant mares were treated with a nonsteroidal anti-inflammatory drug, flunixin meglumine, to inhibit the synthesis of PGF2 alpha caused by Salmonella typhimurium endotoxin given IV. Flunixin meglumine was administered either before injection of the endotoxin (group 1, -10 min; n = 7), or after endotoxin injection into the mares (group 2, 1 hour, n = 3; group 3, 2 hours, n = 3); 12 pregnant mares (group 4) were given only S typhimurium endotoxin. In group 4, the secretion of PGF2 alpha, as determined by plasma 15-keto-13,14-dihydro-PGF2 alpha concentrations, was biphasic, initially peaking at 30 minutes followed by a second, larger peak approximately 105 minutes after the endotoxin was given IV. When flunixin meglumine was administered at -10 minutes, synthesis of PGF2 alpha was inhibited for several hours, after administration of flunixin meglumine at 1 hour, the second secretory surge of PGF2 alpha was blocked, and administration of the drug at 2 hours did not substantially modify the secretion of PGF2 alpha. Plasma progesterone concentrations were unchanged after endotoxin injections were given in group 1. In group 2, progesterone values decreased less than 2 ng/ml and remained low for several days. In group 3 and group 4, progesterone concentrations decreased to values less than 0.5 ng/ml by 48 hours after endotoxin injections were given.(ABSTRACT TRUNCATED AT 250 WORDS)

Thirty pregnant mares were assigned to 3 groups: Group 1 (n = 10) mares served as controls; Group 2 (n = 10) mares were treated with altrenogest (44 mg/day) from Day 16 to 80 and Group 3 (n = 10) mares were treated with a luteolytic dose of PGF2 alpha on Day 16 followed by altrenogest (44 mg/day) until Day 80. Concentrations of progesterone and chorionic gonadotrophin (CG) in plasma and oestrogen conjugate (OC) in urine were determined between Days 16 and 80 of gestation. In Group 3, complete luteolysis occurred in all 10 mares following administration of PGF2 alpha. Six of the 10 mares did not have an active CL from Day 16 until after Day 60 of pregnancy (Group 3a); the remaining 4 mares developed a new CL on Days 32, 40, 43 and 49 of pregnancy (Group 3b). In Groups 1 and 2, an increase in oestrogen secretion was observed between (Group 3b). In Groups 1 and 2, an increase in oestrogen secretion was observed between Days 35 and 40. In Group 3a, OC concentrations in the absence of an active CL were significantly lower than in Groups 1 and 2, and oestrogen secretion did not increase between Days 20 and 50. In Group 3b, OC concentrations remained low in the absence of an active CL. After the onset of CG secretion, OC concentrations increased to values similar to Groups 1 and 2 in conjunction with the development of a new CL. The results of this experiment demonstrate that the increase in oestrogen secretion observed between Days 35 and 50 of pregnancy requires the presence of an active CL.(ABSTRACT TRUNCATED AT 250 WORDS)

A direct enzyme immunoassay was developed to measure conjugated oestrogens in the plasma of pregnant mares. The antibody was produced in rabbits using oestrone-3-glucuronide (E1G) conjugated to bovine serum albumin. The enzyme conjugate was E1G conjugated to horseradish peroxidase. A sharp increase in plasma E1G concentrations occurred between Days 35 and 40 of gestation. Values declined slightly to Day 45, remained relatively constant to around Day 70 and rose sharply thereafter. Fetal death before Day 35 had no effect on plasma concentrations of E1G. Fetal death after Day 35 in conjunction with endotoxin-induced regression of the corpus luteum (CL) resulted in a decrease in plasma E1G levels to non-pregnant values within 3-4 days. Endotoxaemia without fetal death between Days 35 and 70 resulted in marked, but transient, decreases in plasma E1G concentrations. Fetal death without CL regression after Day 35 did not produce an immediate decline in plasma E1G concentrations and existing levels were maintained for 10-14 days. These findings indicate that between Days 35 and 70 of pregnancy, plasma E1G concentrations are not directly correlated with fetal viability; they appear to reflect increased oestrogen secretion by the CL under the influence of chorionic gonadotropin. After Day 70, E1G concentrations begin to reflect directly the production of oestrogen by the developing feto-placental unit.

Stallions can experience an increase in testicular temperature from bouts of fever or from injury to the testes. In species other than the horse, increased temperature models have been used to study testicular degeneration. This study was undertaken to examine the effects of increased testicular temperature on spermatogenesis in the stallion as measured by semen evaluation. The results of this investigation demonstrate that increased testicular temperature is associated with significant transitory alterations in the routine semen evaluation of the stallion. The duration of increased testicular temperature affected the degree of change observed. The temporal nature and degree of the change in the semen evaluation is described. Predictive formulae for expected changes in the semen evaluation have been derived.

Oestrogen secretion was determined by oestrogen conjugate (EC) analysis of urine in three groups of pregnant mares: Group I (N = 6), animals ovariectomized on Day 18-19 of gestation with pregnancy maintained by daily administration of an oral progestagen, altrenogest; Group II (N = 9), untreated, pregnant mares; Group III (N = 5) intact, pregnant mares treated daily with altrenogest. The mean EC concentrations in the ovariectomized mares in Group I increased in a constant linear manner from 17 ng/mg Cr on Day 20 to 291 ng/mg Cr on Day 70, with no apparent surge in oestrogen secretion around Day 39. Mean EC concentrations on Days 33, 39 and 44 were respectively 41, 48, and 73 ng/mg Cr. In the intact mares in Groups II and III (shown in parentheses), the mean urinary EC concentrations were 201 (171) ng/mg Cr between Days 20 and 33 of gestation, increased rapidly from 172 (77) ng/mg Cr on Day 33 to a peak of 1066 (895) ng/mg Cr on Day 39, followed by a decline to 637 (719) ng/mg Cr on Day 44. After Day 44, EC concentrations continued to increase in a linear manner to 1191 (842) ng/mg Cr on Day 70. The mean EC concentrations between Days 20 and 70 in Group I were significantly (P less than 0.05) lower than in mares in Groups II and III. EC concentrations in Group III mares were significantly lower (P less than 0.05) than in Group II mares between Days 28 and 34.(ABSTRACT TRUNCATED AT 250 WORDS)

The infusion of Salmonella typhimurium endotoxin into pregnant mares resulted in a biphasic release pattern of PGF-2 alpha as determined by 15-keto-13,14-dihydro-PGF-2 alpha concentrations. The initial phase of 1 h duration was followed by accentuated release by 2 h after infusion; concentrations reached basal levels by 6 h. In 7 mares at 23, 26, 29, 33, 36, 53 and 55 days of gestation, fetal death occurred between 36 and 120 h after infusion; 12 mares at 46, 51, 56, 59, 65, 71, 73, 85, 103, 138, 283 and 318 days of gestation did not abort after endotoxin infusion. Luteal activity was compromised in all mares by 9 h after infusion. Progesterone concentrations were consistently lower in mares that aborted (1-2 ng/ml) than in those that did not abort. Mares therefore appear to be vulnerable to fetal loss by a clinical syndrome induced by Salmonella typhimurium endotoxin until about 50-60 days of gestation.