We evaluated the mechanism of action by the phytoestrogen genistein in the prepubertal rat uterus, when administered pharmacologically or physiologically. Female rats were injected with genistein (500 μg/g body weight), estradiol benzoate (EB) (500 ng/g body weight) or vehicle, dimethylsulfoxide (DMSO), on days 16, 18, and 20 postnatal. In 21-day-old rats, both compounds increased circulating estradiol and decreased progesterone concentrations. Uterine estrogen receptor alpha (ER-α) and androgen receptor (AR) proteins were reduced, and progesterone receptors (PR) were increased, as measured by western blot analyses. Immunohistochemistry for ER-α was confirmatory. Reverse transcription-polymerase chain reaction (RT-PCR) analyses indicated a decrease in ER-α, but not in ER-β, PR and AR mRNA levels following genistein treatment. In prepubertal rats exposed perinatally to 250 mg genistein per kg AIN-76A diet or 250 μg estradiol per kg diet, uterine ER-α, AR, and PR proteins were not altered significantly. We conclude that pharmacologic, but not physiologic concentrations of genistein can modulate sex steroid receptor expression in the rat uterus. Copyright © 2001 Elsevier Science Ireland Ltd.