Here we show that genistein, through an estrogen receptor-mediated action, modulates gene expression in the mouse testis throughout development. Genistein passed from the lactating mother to the suckling offspring at levels sufficient to activate gene expression in the testis of the pups. Testis are already responsive to genistein as well as to estradiol at day 14.5 of fetal development. Activation of luciferase correlates with an activation of cell proliferation. In conclusion, our results show that genistein affects reproductive organs of male mice at all developmental ages.