The importance of epoxide hydrolase in preventing styrene 7,8-oxide-induced hepatoxicity was studied in isolated perfused rat livers depleted of GSH. GSH depletion was accomplished by treating the rats with diethyl maleate 45 min before surgical removal of their livers. Diethyl maleate itself caused mild hepatotoxicity that was observed histologically and measured biochemically by the release of hepatic transaminase enzymes into the circulation. GSH transferase activity was decreased in perfused livers from diethyl maleate-treated rats as shown by the persistence of circulating styrene oxide compared with that seen in experiments with livers from control animals. In the absence of GSH transferase activity, the rate of styrene oxide biotransformation by epoxide hydrolase was sufficient to prevent measurable hepatotoxicity, up to epoxide concentrations tolerated by livers from control rats. The administration of 500 mumol of styrene oxide to perfused livers from diethyl maleate-treated rats caused periportal necrosis and extensive covalent binding of styrene oxide-derived radioactivity to tissue protein. These effects, however, were no more severe than those seen in perfused livers from control animals given 500 mumol of styrene oxide. Due to high GSH transferase activity in perfused livers from untreated rats, the capacity of epoxide hydrolase to detoxify styrene oxide is difficult to measure in this system. The detoxication potential of epoxide hydrolase was clearly demonstrated in this study with GSH-depleted liver preparations.