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Ovariectomy of hamsters on day 1 or day 4 of pregnancy without steroid replacement resulted in complete failure of implantation. This was, however, induced by daily subcutaneous (s. c.) injection of either 1 or 2 mg progesterone. Although implantation could occur in the absence of oestrogen, greater numbers of eggs implanted in its presence after ovariectomy on day 1. Oestrogen administration (1 μg/hamster/day s. c. of oestrone) also increased the proportion of embryonic development to day 14 in the ovariectomized, progesterone-treated animal.

The deciduomal response to uterine trauma in rats ovariectomized on day 2 or 4 of pseudopregnancy and given oestrone, 1 μg, progesterone, 2 mg, on various days thereafter has been compared with that in intact pseudopregnant controls. Trauma was effected by inserting a sterile silk thread into the lumen of one uterine horn of each animal on day 5 of pseudopregnancy and the response was evaluated at autopsy on day 9**. Significant differences in response were observed between rats ovariectomized on days 2 and 4, respectively, of pseudopregnancy. Progesterone whether given alone or with oestrone to rats ovariectomized on day 2 of pseudopregnancy produced an increase in weight of the traumatized horn not significantly smaller than that produced in intact controls. In animals given progesterone alone, however, the ratio of weight of the traumatized to untraumatized horns (T/U) was significantly smaller than control. In animals ovariectomized on day 4 of pseudopregnancy a different situation obtained. Progesterone alone from ovariectomy to autopsy produced only a moderate increase in the weight of the traumatized horn and a ratio T/U half that seen in control animals. Thus, in such animals both oestrone, given at least on day 4, and progesterone were needed to produce a deciduomal response not different from control.

Deciduomal growth was stimulated in rats by inserting a silk thread into the lumen of one uterine horn on day 5 of pseudopregnancy. The degree of response was evaluated at autopsy on day 9. Deciduomal growth under such conditions was found to be reduced by the administration of methallibure (I. C. I. 33,828), 100 mg/kg p. o., on days 2 to 4 or – to a lesser extent – on days 4 to 6. Additional treatments with oestrone, 1 μg/rat and/or progesterone, 2 mg/rat, given subcutaneously on various days during and after administration of methallibure on days 2 to 4, failed to restore deciduomal growth to normal levels. This was achieved, however, when methallibure was given on days 4 to 6, by additional treatment with progesterone on days 4 to 7 combined with oestrone on days 4, 4 and 5, or 4 to 7, although neither progesterone nor oestrone was effective alone. These results indicate that the administration of methallibure reduces the output of both oestrogen and progesterone from the ovary to levels below those required for normal deciduomal growth. They also suggest that this effect persists for more than 2 days after administration of the compound is stopped.