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Summary. Ovariectomized pregnant rabbits given sufficient proges- terone maintained their pregnancies well but, after kindling, only a low percentage of young were found alive. Addition of oestradiol to the treat- ment regimen did not alter the effectiveness of progesterone in prevent- ing abortion but increased the percentage of young found alive after parturition. The beneficial effect of oestrogen appeared to be related to a reduced incidence of prolonged pregnancies and prolonged labours, and suggested a r\l=o^\lefor oestrogen in the successful termination of pregnancy. In the rabbit, the ovary is essential throughout gestation for the maintenance of pregnancy. In ovariectomized rabbits, pregnancy can be maintained by ad¬ ministration of progesterone and its withdrawal is followed by parturition. If, however, the objective is to simulate a normal pregnancy with the birth of viable offspring at full-term, the results so far obtained with progesterone replacement therapy appear to be poor, yielding few viable offspring. If such poor results could be confirmed, they might be shown to reflect a basic in¬ adequacy of progesterone replacement therapy or merely an inappropriate dose level. It also seems possible that the addition of oestrogen to progesterone might influence the outcome of pregnancy, since oestrogen should also be deficient in the ovariectomized animal. In earlier studies (Courrier & Kehl, 1938; Pincus & Werthessen, 1938; Allen & Heckel, 1939) of hormone replace¬ ment in ovariectomized rabbits, progesterone treatment has on occasion been supplemented with oestrogens, but usually only briefly, early in gestation, or only in one or two animals. More recently, Csapo (1969) has reported obser¬ vations in pregnant rats ovariectomized 48 hr before term. They found that none, or only a fraction of the litters of these animals were born, the remaining fetuses eventually dying in utero. Oestrogen substitution therapy, given as a single dose under various regimens, permitted the delivery of normal litters at term. In our studies, trials were made in ovariectomized pregnant rabbits with progesterone in varying doses, with or without added oestrogen. The results

IMPROVED VIABILITY OF OFFSPRING WITH OESTROGEN SUPPLEMENTATION OF OVARIECTOMIZED RABBITS MAINTAINED ON PROGESTERONE