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Summary.Preovulatory bovine follices (n= 73) were collected at different times after the onset of oestrus until shortly before ovulation, which occurred at 24 ± 1 ·4 h after the peak concentration of LH in the peripheral blood. Non-atretic antral follicles (n= 9) of 15–19 mm were also collected from cows during the luteal phase of the oestrous cycle. Follicular fluid concentrations of dehydroepiandrosterone, androstenedione and oestrone, and of LH, FSH and prolactin were compared in 2-h periods relative to the LH plasma peak. Before the LH surge the concentrations of the steroids were much higher than in non-atretic luteal-phase follicles of similar size. From 0 to 6 h after the LH peak the steroid concentrations decreased sharply to remain low until ovulation; only that of androstenedione increased again after 14 h to remain constant. The ratio between the concentrations of androstenedione and dehydroepiandrosterone remained constant until 14 h after the LH peak; at 14 h it increased about 4-fold and remained high until ovulation. The ratio between the oestrone and androstenedione concentration increased gradually to a 10-fold higher value until at 14 h an abrupt decrease was observed. These changes indicate that after the LH peak androgen production is directly inhibited and, at a slower rate, the aromatizing activity. Androstenedione appeared to be the major aromatase substrate.Before the plasma LH peak the follicular fluid concentration of FSH was higher than in luteal-phase follicles; the concentrations of LH and prolactin were not different from those in luteal-phase follicles. About 4 h after the LH plasma peak the LH concentration in follicular fluid reached a maximum, which was one seventh of that in peripheral blood; it remained elevated until 20 h. The FSH concentration was higher after the LH plasma peak than before; just before ovulation it was still 2-fold higher. The concentration of prolactin fluctuated throughout the preovulatory development from the onset of oestrus until ovulation. It is suggested that oestradiol biosynthesis in bovine preovulatory follicles is terminated by an inhibitory action of the preovulatory LH peak on androgen production.

Abstract. Lactating sows (5 per group) were given no treatment or 10 mg bromocriptine orally twice daily from day 14 to 22 after parturition. Both groups were weaned at day 22. Frequent bleedings at 10 min intervals for 6 h were performed at 12, 16, 20 and 24 days after parturition. Prolactin and LH levels were measured in the collected blood samples by radioimmunoassays. Reduction of Prl by bromocriptine increased mean LH levels significantly and a further significant increase was observed after weaning. From the results it is concluded that in the lactating sow both Prl per se and the suckling stimulus are involved in suppression of LH levels.

Abstract. Insulin tolerance tests were carried out in chronically catheterised foetal pigs. The experiment was carried out 7 days after catheterisation. The foetuses were, therefore, considered to be free from the effects of anaesthetics and stress. Under these conditions growth hormone levels were high compared with post-natal growth hormone levels but, under the favourable conditions in this study, both growth hormone and ACTH levels were lower than those found in anaesthetised or stressed foetuses. By contrast, cortisol levels were somewhat higher than those mentioned in previous reports. Somatomedin activity measured by post-natal cartilage bioassay was low.Following iv insulin administration there was a marked depression in plasma glucose (P< 0.01), an elevation in growth hormone (P< 0.05) and an increase in ACTH (P< 0.01). Levels of cortisol and somatomedins did not change significantly. From these data it is concluded that insulin is an hypoglycaemic factor in the foetal pig and that, for the most part, the foetal pig pituitary responds to an insulin challenge in a similar way to the post-natal pig.

Summary.Normally cyclic heifers received 2500 i.u. PMSG i.m. at Day 10 of the oestrous cycle and 15 mg prostaglandin (PG) i.m. 48 h later. From 30 h after PG the LH concentration in the peripheral blood was estimated every hour using a rapid RIA method which allowed the LH concentration to be known within 4 h. Monoclonal antibody against PMSG was injected in the jugular vein of 29 heifers at 4·8 h after the maximum of the preovulatory LH peak; 28 heifers were not treated with anti-PMSG (controls). Peripheral blood concentrations of PMSG, LH, progesterone and oestradiol were compared. Ovaries were collected by ovariectomy at fixed times, 22–30 h after the LH peak, and numbers were counted of small (2–10 mm), large (> 10 mm) and ovulated follicles, and of follicles with a stigma.In anti-PMSG-treated cows, the PMSG concentration fell sharply to non-detectable levels within 2 h of the treatment, indicating that PMSG was neutralized in these cows at the onset of final follicular maturation. In all cows, the concentration of oestradiol showed a significant decrease at about 8 h after the LH peak. After anti-PMSG treatment ovulations took place from 24 until 30 h after the LH peak, whereas in control cows follicles had already ovulated at or before 22 h and ovulations continued until 30 h. At 30 h 90% of the follicles had ovulated in anti-PMSG-treated cows vs72% in the controls, resulting in 15 and 8 ovulations per cow respectively (P< 0·05). Also, administration of monoclonal antibody against PMSG synchronized final follicular maturation and shortened the period of multiple ovulations.In conclusion, neutralization of PMSG shortly after the preovulatory LH peak suppresses adverse effects of PMSG on final follicular maturation, leading to an almost 2-fold increase of the ovulation rate.

Summary.Six heifers were injected i.m. with 2500 i.u. PMSG followed by 15 mg prostaglandin 48 h later. Serial blood samples were collected through a catheter in the caudal vena cava every 10 min for 8 h on Day 10 (7 h after PMSG administration), during luteal regression (7 h after prostaglandin administration) and on the day thereafter. Four normally cyclic heifers served as a control group. Concentrations of progesterone, androstenedione, oestradiol, LH, FSH, and PMSG in the vena cava samples were measured and the frequency and amplitudes of episodic pulses of all hormones were estimated except for PMSG. Ovaries were collected by ovariectomy at 50 h after onset of luteal regression to determine the number of preovulatory follicles (non-atretic follicles ⩾ 10 mm).Stimulation of follicular growth by administration of PMSG resulted in the following effects on the secretion of steroids and endogenous gonadotrophins. (1) There were no alterations in progesterone concentration and the amplitude and frequency of episodic pulses. Mean (±s.e.m.) concentrations were 54·1 ± 5·8, 19·1 ± 3·1 and 3·4 ± 0·9 nmol/l on Day 10 (L), during luteal regression (LR) and on the day thereafter (F) respectively. (2) There were no alterations in the episodic secretion patterns of androstenedione. Mean concentrations were 0·20 ± 0·02, 0·15 ± 0·02 and 0·11 ± 0·02 nmol/l for the L, LR and F periods respectively. (3) There was an increase in oestradiol concentration from 17·1 ± 3·0 pmol/l during the L period to 233·7 ± 86·4 pmol/l during the F period. Pulse amplitude was enhanced compared to corresponding periods in control animals whereas pulse frequency remained the same. The oestradiol concentration was significantly correlated with the number of preovulatory follicles (r= 0·82, P< 0·05). (4) There was a suppression of the frequency of episodic LH pulses (/8 h) during the LR (3·2 ± 0·7) and F (4·3 ± 0·4) periods compared to corresponding periods in control heifers (9·5 ± 0·9 and 7·0 ± 1·5 respectively). The preovulatory LH peak occurred earlier in 4 of 6 treated heifers. (5) There was a suppression of FSH concentrations, pulse amplitude and frequency during the LR and F (17·4 ± 0·9 mg/l, 4·7 ± 0·8 μg/l and 7·5 ± 0·4 pulses/8 h) periods compared to the corresponding F-period values (35·6 ± 6·2 mg/l, 9·8 ± 1·6 μg/l and 9·3 ± 0·3 pulses/8 h) in control heifers.It is concluded that the increased number of preovulatory follicles after PMSG administration severely suppresses endogenous FSH secretion by means of enhanced concentrations of oestradiol and probably also of inhibin.Keywords:superovulation; PMSG; steroids; gonadotrophins; cow