Peroxisome proliferators have been shown to diminish acetaminophen (APAP) hepatotoxicity (Biochem. Pharmacol. 43, 1395, 1992). To investigate the mechanistic basis for this protection CD-1 male mice were given corn oil or 500 mg clofibrate (CFB)/kg, ip, daily for 10 days. They were then fasted overnight and either killed without challenge or at 4 or 12 hr after challenge with 800 mg APAP/kg (in 50% propylene glycol). At 12 hr, hepatotoxicity was evidenced by elevated plasma sorbitol dehydrogenase and histopathology in corn oil but not in CFB-pretreated mice. At 4 hr after APAP treatment, hepatic glutathione (GSH) depletion and selective arylation of the major APAP target proteins were both greatly diminished by CFB pretreatment. Western blot analysis with the anti-58 antibody of liver cytosol from unchallenged mice showed no apparent changes in the levels of the 58-kDa major APAP target protein with CFB treatment. These findings suggest that protection could be the result of diminished net availability of generated electrophile. In vitro, measurements indicated that the specific activity in microsomes for APAP oxidation by cytochrome P450 was not changed by CFB treatment; whereas GSH S-transferase activity in cytosol was decreased by 25%. Pretreatment with CFB also produced a significant elevation in hepatic GSH. These studies indicate that protection by CFB might result from increased availability of hepatic GSH which could trap APAP electrophile nonenzymatically, thereby decreasing covalent binding and preventing toxicity.