Biochanin A, a nonsteroidal estrogen present in plants, has been suggested in epidemiological studies to be associated with a reduced risk of hormone-related cancers. In this study, we examined the effects of biochanin A on the cell growth and gene expression of an estrogen-responsive pituitary cell line GH 3 . Results showed that biochanin A inhibited growth of GH 3 cells in a dose-dependent manner and at physiologically relevant concentrations. At concentrations of 1 and 5 µg/mL, inhibition of cell growth was observed. The gene expressions of estrogen receptor (ER) mRNA and an estrogen responsive gene, the progesterone receptor (PR), were down-regulated by the biochanin A treatment. In addition, co-administration of antiestrogen ICI182,780 blocked this biochanin A induced down-regulation of the ER and PR mRNA level. However, the expression level of rat prolactin (rPRL) gene, one of the typical estrogen responsive genes, was not regulated by biochanin A. These results indicated that biochanin A at physiologically achieveable concen-trations was able to inhibit cell growth of estrogen-responsive rat pituitary cells. In addition, dietary-derived biochanin A compounds exhibit diverse estrogenic and antiestrogenic effects on estrogen-related gene expression, suggesting gene-specific regulation.