Excess production of H2O2 has been implicated in oncogenesis. The object of the present study was twofold: first, to determine the influence of chronic estradiol (E2) on the activities of selected hepatic antioxidant enzymes in female ACI rats, a strain that is highly sensitive to the induction of estrogen dependent mammary tumors; secondly, to evaluate the actions of dietary clofibrate, a peroxisome proliferator, on activities of these enzymes in control and E2-treated ACI rats. Enzymes selected for study were: NAD(P)H quinone oxidoreductase (NQO1), glutathione S-transferase (GST) and glutathione peroxidase (GPx). Cytosolic catalase (CAT) was also measured as an index of peroxisome proferation in control and E2- treated animals. E2 was administered chronically over 6 and 12 week periods from cholesterol pellet implants containing either 1 or 3 mg E2. Animals were fed AIN-76A diets with or without 0.4% clofibrate over the experimental period. NQO1 and GST but not GPx were induced to varying degrees (NQO1 about 300%, and GST about 45-97%) by chronic E2-treatment. E2-induced increases in these activities were completely prevented in rats exposed to dietary clofibrate. Dietary clofibrate also caused slight but significant reductions in baseline activities of NQO1, GST and GPx in control animals. Serum E2 levels, increased approximately 540% in a dose-dependent manner, and were not altered by dietary clofibrate. It is concluded that chronic E2 treatment markedly induces several important hepatic antioxidant enzymes in female ACI rats, and induction of these activities by E2 is inhibited completely by dietary clofibrate. Both of these actions have the potential to markedly influence the profile of E2 metabolites exported from the liver to E2 sensitive extrahepatic tissues and influence the initiation and progression of hormone-dependent tumors.