Neonatal treatment of rodents with the widely used antiestrogen tamoxifen causes endometrial cancer and reproductive tract lesions reminiscent of the diethylstilbestrol (DES) syndrome. To evaluate the initial alterations induced in the developing uterus by tamoxifen or DES, neonatal Sprague-Dawley rat pups received 100 micrograms of tamoxifen (Group 1), 1 microgram of DES (Group 2), or vehicle (Group 3) subcutaneously on days 1 through 5, and their uteri were studied by light microscopy, 5-bromo-2' deoxyuridine immunohistochemistry, and computer-based morphometry. At Postnatal Day 6, epithelial hypertrophy (184.3% and 237.9% of controls) and myometrial thickening (151.9% and 180.0%) accounted for the uterotrophic effects of tamoxifen and DES. Evidence of secretory activity in epithelial cells, reduction of the epithelial BrdU-labeling index to 18.1% (tamoxifen) and 41.1% (DES) of controls, premature endometrial and myometrial differentiation, and the presence of eosinophils in both treatment groups suggested that tamoxifen exerted a DES-like estrogenic action on the developing uterus. These findings indicate that immediate epithelial and stromal-myometrial uterine alterations are found at Postnatal Day 6 after neonatal tamoxifen treatment.