Tamoxifen can exert its effects through the competitive inhibition of estrogen receptors or other mechanisms. HepG2 cells lacking estrogen receptors and engineered to overexpress CYP3A4, the most important CYP to metabolize the drug, appear to be a good model to study the effects of tamoxifen metabolites. Tamoxifen altered cell cycle of transduced HepG2 cells, decreased G0/G1 cell numbers, diminished proliferation index and induced cell death mostly in cells overexpressing CYP3A4 but was without significant effect on cytotoxicity or proliferation of cells engineered to overexpress CYP2E1 or on empty vector transfected cells. Tamoxifen did not change MDR1 levels irrespectively on CYP450s expression, but inhibited by approximately 50% p-gp functions in all cell types. Drug treatment significantly increased dehydroepiandrosterone sulfotransferase activity and sulfotransferase inhibition significantly decreased tamoxifen cytotoxicity. Our results support the view that metabolic activation of tamoxifen in liver cells may proceed via CYP450-mediated metabolism and subsequent sulfotransferase-mediated activation and point to the role of CYP3A4 and dehydroepiandrosterone sulfotransferase in adverse tamoxifen effects.