When rats ovariectomized at 3 days of age and given a single injection of 1.25 mg testosterone propionate on the next day became 60 days old, they were given 3 daily injections of 0.2 mug estradiol-17 beta followed by 7 daily combined injections of 2 mg progesterone and 0.2 mug estradiol. Incidence of deciduomata in reaction to uterine trauma applied on the 4th day of the progesterone-estradiol injections was almost as high as that in neonatally ovariectomized, non-androgenized rats, but the response was significantly smaller in size in androgenized rats than in non-androgenized animals. If females similarly operated on were given injections of 0.1 mug estradiol for 30 days prior to 7 daily injections of progesterone-estradiol, deciduoma formation in androgenized rats was markedly reduced in both incidence and size of the response. In non-androgenized group, deciduoma formation was not significantly affected by chronic administration of estradiol.Accordingly, it is likely that, although androgen injected during neonatal life is responsible for the reduction of uterine responsiveness in androgen-sterilized rats (Takewaki and Ohta, 1974) continued exposure of the uterus to estrogen may play a co-operative role in the event.