Adult female rats which had been ovariectomized on Day 1, 5, 10 or 20 of age invariably formed deciduomata in response to uterine trauma or an intraluminal oil-instillation given after injections of estradiol-17 beta for 3 days from Day 60 onward followed by daily injections of progesterone starting 2 days later. If the magnitude of the deciduomal response was estimated by the decidual cell reaction index, the responses were larger in these rats than in the animals ovariectomized as adults. Two groups of rats ovariectomized on Days 1 and 10 were transplanted subcutaneously with one ovary on Day 10. When subjected to deciduogenic stimuli, after removal of the grafts on the first day of estrus occurring after Day 60 followed by hormonal sensitization of the endometrium, the 2 groups of rats elicited deciduomal responses approximately the same in magnitude. These findings indicate that the neonatal ovary has little effect on the development and differentiation of the rat uterus.