Previous work has indicated that metabolic activation of tamoxifen in rat liver cells involves cytochrome P450-mediated α-hydroxylation,followed by sulfate ester formation, mediated by hydroxysteroid sulfotransferase a (rHSTa), a member of the SULT2A subfamily, which efficiently metabolizes dehydroepiandrosterone. Because it is known that the expression of rHSTa and other SULT2A forms is substantially higher in female rats than in males, it might be predicted that tamoxifen would be a more potent liver carcinogen in females than in males. Yet tamoxifen has been shown to be equipotent in both sexes. To investigate this paradox, primary cultures of hepatocytes were prepared from Fischer F-344 rats and treated with tamoxifen (10 μm) or α-hydroxytamoxifen (1μ m). Rats were also treated with tamoxifen daily by gavage (0.12 mmol/kg/day) for up to 14 days. DNA was isolated from hepatocytes and liver and analyzed by ³²P-postlabeling. Liver cytosol fractions were prepared and analyzed for dehydroepiandrosterone sulfotransferase activity and SULT2A protein levels. In tamoxifen-treated hepatocytes and after a single dose of tamoxifen in vivo, DNA adduct formation in male cells was significantly lower than in female cells, 11- and 6-fold,respectively. However, with increasing daily doses of rats with tamoxifen, the adduct level in males increased to a level 89% of that in females by 14 days. Dehydroepiandrosterone sulfotransferase activity in male rat liver cytosols was only 17% of the activity of female cytosols after one dose of tamoxifen but 64% after 14 days of exposure to the compound. This increase in activity correlated with increases in the levels of SULT2A protein, detected by Western blotting. Western blotting did not allow the unambiguous identification of the induced SULT2A form(s). However, by using a specific reverse transcriptase/PCR technique, it was found that it was primarily rHSTa that was induced. Thus, after prolonged exposure to tamoxifen, DNA adduct formation and rHSTa expression in males are significantly closer to the levels in females than they are after initial exposure. These changes explain the similar susceptibility of male and female rats to tamoxifen carcinogenesis.