Eugenol is widely used as a food flavoring agent and a dental analgesic. Mice treated with eugenol (400-600 mg/kg, po) in combination with an inhibitor of glutathione (GSH) synthesis, buthionine sulfoximine (BSO; 1 hr before eugenol, 4 mmol/kg, ip) developed hepatotoxicity characterized by increases in relative liver weight and serum GPT, hepatic congestion, and centrilobular necrosis of hepatocytes. Eugenol (up to 600 mg/kg) alone produced no hepatotoxicity. Drug metabolism inhibitors such as carbon disulfide, methoxsalen, and piperonyl butoxide prevented or significantly reduced the hepatotoxic effect of eugenol given in combination with BSO. On the other hand, pretreatment with phenobarbital (PB) increased the hepatotoxicity. These results suggest that eugenol is activated by a cytochrome-P-450-dependent metabolic reaction and that the liver injury is caused by inadequate rates of detoxification of the resulting metabolite in mice depleted of hepatic GSH by BSO treatment.