We have previously shown that rat uterine gland genesis occurs rapidly and synchronously between postnatal days 9-15. Exogenous estrogens either stimulate or inhibit gland genesis depending on dose and age at administration. We therefore examined the developmental effects of the triphenylethylene antiestrogen tamoxifen, which exhibits both estrogen agonist and antagonist properties, in the postnatal rat uterus. Tamoxifen administered sc in oil on postnatal days 1-5 or days 10-14 caused dose-related inhibition of uterine gland genesis which persisted to day 26 or day 60, respectively. Tamoxifen administered on postnatal days 20-24, which is after the age of normal gland genesis, did not alter the number of preexisting glands. A 24-h exposure to tamoxifen inhibited 17 beta-estradiol (E2)-induced ornithine decarboxylase (ODC) activity measured 6 h after E2 administration in 14-day-old rats. Treatment with tamoxifen before or during the period of gland genesis also reduced uterine responsiveness to a single dose of E2 as measured by both uterine weight gain (after a 24-h exposure on days 14, 19, 22, and 26) and the pattern of E2-induced ODC activity in 26-day-old rats. Control rats respond to E2 with peaks of ODC activity at 6 and 18 h after administration. Treatment with tamoxifen on either postnatal days 1-5 or 10-14 reduced the 18-h peak to approximately half of controls but did not affect the 6-h E2-induced ODC peak. Analysis of both nuclear and translocatable cytosol estrogen receptor in uteri from 26-day-old rats indicate that neither the dissociation constant (KD) nor the number of binding sites was affected by tamoxifen treatment on postnatal days 1-5 or 10-14.