Female rats of the T strain were given single daily injections of 100 micrograms and 200 micrograms of tamoxifen (Tx) and MER-25 (MER) for 5 days beginning on the day of birth (DAY 1). When sacrificed on Day 60, the Tx-treated rats (Tx rats) exhibited continued vaginal diestrus, whereas the females given MER or the vehicle alone showed regular estrous cycles. Ovaries from Tx rats were polyfollicular without corpora lutea, while those from MER rats, as well as from the controls given the vehicle alone, invariably contained both follicles and corpora lutea. In Tx rats, the uteri underwent atrophy, containing few uterine glands in an endometrium largely occupied by fibroblasts. Decidual response of the uterus to intraluminal oil instillation was markedly reduced in Tx rats given an appropriate regimen of progesterone and estradiol injections following ovariectomy on Day 60. By contrast, MER given neonatally had little effects on decidualization. Since ovariectomy on Day 10 brought about no amelioration of the decidualization in Tx rats, it is suggested that the lowered deciduogenic responsiveness to the instillation was caused by a direct action of Tx on the uterus of neonatal rats rather than by a Tx-induced alteration of hypothalamo-hypophyseal-ovarian system. Differences in effect on the female reproductive system between Tx and MER were discussed.