This study used the neonatal mouse model to determine if early exposure of female mice to coumestrol, a plant estrogen, would result in reproductive-tract alterations similar to those seen after neonatal exposure to diethylstilbestrol (DES). Newborn female C57BL/Crgl mice were given daily subcutaneous injections of 0.08 micrograms DES or 100 micrograms coumestrol in 0.005 ml dimethyl sulfoxide (DMSO), or DMSO alone, or were untreated, for the first 5 d of life. The doses chosen were equivalent in biological activity based on published uterine bioassay data (using young adult mice). Observations were made twice daily for 1.5 mo to determine the times of eye and complete vaginal opening. Half of the animals were ovariectomized at 40 d of age. Vaginal lavages were examined for 30 consecutive d beginning both at 2 and at 5 mo of age. DES and coumestrol significantly advanced the time of complete vaginal opening and induced a comparable degree of ovary-independent persistent vaginal cornification. In addition, coumestrol resulted in the occurrence of hemorrhagic ovarian follicles.