Genistein and ethinyl estradiol (EE(2)) were examined in multigenerational reproductive and 2-yr chronic toxicity studies with different exposure durations across generations F(0) through F(4). Sprague-Dawley rats were exposed to genistein (0, 5, 100, or 500 ppm) or EE(2) (0, 2, 10, or 50 ppb). Effects in the male mammary gland are described here. In the multigeneration studies, mammary hyperplasia was induced by both compounds; the chronic studies had a lower incidence, without proportionate neoplasia. Sexual dimorphism (predominant tubuloalveolar growth in females and lobuloalveolar in males) was retained without feminization in high dose genistein or EE(2). In the continuously exposed generations, mammary hyperplasia was sustained but not amplified, appeared morphologically similar across all generations, and was not carried over into unexposed offspring of previously exposed generations. The hyperplasia in male rats was similar whether induced by genistein or EE(2). Results substantiate and extend previous reports that mammary gland hyperplasia in the male rat is one of the most sensitive markers of estrogenic endocrine disruption.