Endocrine disruptors are a diverse group of chemicals that alter the functions of the endocrine system. A large proportion of endocrine disruptors have estrogenic effects, thus are called environmental estrogens. In the present study, an estrogen (E₂) responsive rat pituitary cell line, MtT/E-2, was employed to examine 1) the potency of several endocrine disruptors including bisphenol A (BPA), o, p'-DDD, methoxychlor, 1, 2, 3, 4, 5, 6-hexachlorocyclohexane (HCH) and dibromoacetic acid (DBAA) in terms of E₂ responsive pituitary cell growth; 2) whether BPA has estrogenic action in vivo causing the growth of MtT/E-2 cells grafted in rats. Binding assays showed the test chemicals were able to compete with ³H-E₂ binding to the estrogen receptor (ER). The compounds also stimulated growth of MtT/E-2 cells at rates corresponding to their ER binding affinity. Their transcription activation of an (ERE)₃-SV40-luciferase reporter in MtT/E-2 cells was comparable to their stimulation of cell growth, with the exception of HCH which showed little induction of cell growth but strong stimulation in ERE dependent transcription activation. MtT/E-2 cells were inoculated into ovariectomized female F344 rats treated with E₂ or BPA. The first tumors were noted at day 22 in the E₂ treated group, at day 25 in the highest dose of BPA group and at day 41 in the control group. These results suggest 1) that the growth assay with MtT/E-2 cells provides simple and sensitive test for detection of estrogenic activity of environmental chemicals; 2) that BPA has estrogenic potency to stimulate E₂ responsive cell growth in vivo as well as in vitro.