Fenbendazole is a broad-spectrum anthelmintic drug widely used to prevent or treat nematode infections in laboratory rodent colonies. Potential interactions between fenbendazole and hepatotoxicants such as acetaminophen are unknown, and this was investigated in this study. Mice were fed a control diet or a diet containing fenbendazole (8-12 mg/kg/day) for 7 days prior to treatment with acetaminophen (300 mg/kg) or phosphate buffered saline. In mice fed a control diet, acetaminophen administration resulted in centrilobular hepatic necrosis and increases in serum transaminases, which were evident within 12 h. Acetaminophen-induced hepatotoxicity was markedly increased in mice fed the fenbendazole-containing diet, as measured histologically and by significant increases in serum transaminase levels. Moreover, in mice fed the fenbendazole-containing diet, but not the control diet, 63% mortality was observed within 24 h of acetaminophen administration. Fenbendazole by itself had no effect on liver histology or serum transaminases. To determine if exaggerated hepatotoxicity was due to alterations in acetaminophen metabolism, we analyzed sera for the presence of free acetaminophen and acetaminophen-glucuronide. We found that there were no differences in acetaminophen turnover. We also measured cytochrome P450 (cyp) 2e1, cyp3a, and cyp1a2 activity. Whereas fenbendazole had no effect on the activity of cyp2e1 or cyp3a, cyp1a2 was suppressed. A prolonged suppression of hepatic glutathione (GSH) was also observed in acetaminophen-treated mice fed the fenbendazole-containing diet when compared with the control diet. These data demonstrate that fenbendazole exacerbates the hepatotoxicity of acetaminophen, an effect that is related to persistent GSH depletion. These findings are novel and suggest a potential drug-drug interaction that should be considered in experimental protocols evaluating mechanisms of hepatotoxicity in rodent colonies treated with fenbendazole.