Diallyl sulfide (DAS), a major flavour component of garlic, is known to modulate drug metabolism and may protect animals from chemically induced toxicity and carcinogenesis. In this study the effects of DAS on the oxidative metabolism and hepatotoxicity induced by acetaminophen (APAP) in rats were investigated. In the hepatotoxicity evaluation of Fischer 344 rats there was a dose-dependent increase in the odds of mortality rate by APAP (P = 0.009); DAS treatment significantly protected rats from APAP-related mortality (P = 0.026). Liver toxicity determined by lactate dehydrogenase activity was significantly increased by APAP treatment (0.75 g/kg). Pretreatment with DAS protected animals from APAP-induced liver toxicity in a time- and dose-dependent fashion. Treatment of DAS (50 mg/kg) 3 hr after APAP dosing significantly (P < 0.05) protected rats from APAP-induced liver toxicity. The metabolism of APAP (50 microM) in vitro was significantly inhibited by DAS (0.3-1 mM) in liver microsomes isolated from F344 rats. As the effect of DAS on APAP-induced hepatotoxicity in vivo was observed only when DAS was administered before or shortly after (< 3 hr) APAP dosing, data suggested that the protective effect of DAS is mainly at the metabolic activation step of APAP. However, the possibility that DAS may also have effects on other drug metabolism systems, such as glutathione (GSH) and glutathione S-transferases, cannot be ruled out.