In vivocovalent binding of halothane biotransformation-reactive intermediates to hepatic protein and lipid was examined in association with the subsequent development of hepatic necrosis in the guinea pig. Oxidative halothane biotransformation was inhibited by the use of deuterated halothane, whereas reductive metabolism was enhanced by low inspired oxygen concentrations. Male outbred Hartley guinea pigs (n = 8) were exposed to either 1% (v/v) halothane or deuterated hatothane—with a fractional inspired O2concentration (FIo2) of 0.40 or 0.10—for 4 h. Livers removed from half of the animals immediately after anesthesia were evaluated for organic fluorine bound to protein and lipid. The remaining animals were evaluated for a hepatotoxic response up to 96 h after exposure. Only guinea pigs that received 1% halothane at an FIo2of 0.40 had centrilobular necrosis develop with significantly increased plasma alanine aminotransferase activities. All other treatment conditions significantly reduced oxidative halothane biotransformation, as indicated by decreased plasma trifluoroacetic add concentrations. These reductions were associated with a significant decrease in organic fluorine bound to hepatic proteins. An FIo2of 0.10 during halothane anesthesia significantly enhanced reductive biotransformation, as indicated by plasma fluoride ion concentrations. This was associated with a significant increase in organic fluoride bound to hepatic lipids. Centriobular necrosis did not develop under these conditions. Thus, covatent binding to subcellular proteins by the trifluoroacetyl add chloride intermediate generated by oxidative halothane biotransformation is implicated as a mechanism of centrilobular necrosis in guinea pigs. Binding to lipids by reductive pathway generated free radicals does not appear to be involved in production of the lesion.