trans-Stilbene exhibits estrogenic activity in an estrogen reporter assay after metabolic activation. In this study, uterotrophic assay was conducted using ovariectornized B6C3F1/Crj female mice treated with trans-stilbene, trans4-hydroxystilbene, trans-4,4'-dihydroxystilbene and diethylstilbestrol. Administration of trans-4-hydroxystilbene, trans-4,4'-dihydroxystilbene and diethylstilbestrol elicited increases in absolute and relative uterus weight. Furthermore, the uterine response caused by the hydroxylated stilbenes was accompanied with an increase in the thickness of epithelial cell layers. trans-Stilbene itself also showed estrogenic activity, affecting uterine weight and causing histological changes of the uterus. This suggests that trans-stilbene is activated to hydroxylated metabolites to exhibit its estrogenic activity. Indeed, trans-hydroxystilbenes were detected in the urine and feces of mice dosed with trans-stilbene. The effect of stilbene derivatives on testis in newborn B6C3F1/Crj mouse was examined. Administration of diethylstilbestrol slightly decreased the testis weight and atrophy of seminiferous tubules. However, trans-stilbene and trans-4-hydroxystilbene affected neither testis weight nor the histological appearance of the testis.