The antiestrogen tamoxifen, administered to newborn female rats, caused gross abnormalities in reproductive development. Vaginal opening was early, cycles were absent and at 4 months of age all ovaries and uteri were atrophic while oviducts showed severe squamous metaplasia with abscess formation. No overt malignancy was detected at this early age, but these results and similar findings with nafoxidine and clomiphene may suggest caution in the use of antiestrogens and similar compounds during possible pregnancy.