The formation of pentane and anaerobic metabolites of halothane (2-chloro-1,1,1-trifluoroethane and 2-chloro-1,1-difluoroethylene) in a mixture of guinea pig liver microsomes and halothane (2-bromo-2-chloro-1,1,1-trifluoroethane) in the presence of NADPH was studied by gas chromatography. Under anaerobic conditions, pentane was formed without halothane and was inhibited by oxygen tension. This anaerobic pentane formation was potentiated 2.5 times by addition of halothane. Halothane-induced pentane formation increased dose-dependently with a halothane concentration of up to 2.1 mmol/liter and then decreased in the presence of increasing concentrations of halothane. Inhibition by a higher substrate was also observed in the formation of anaerobic metabolites of halothane. Antioxidant agents, vitamin E and glutathione, reduced the pentane formation, but did not reduce the anaerobic metabolites of halothane. Metyrapone, an inhibitor of cytochrome P-450, reduced both the pentane and anaerobic metabolites of halothane. These results show halothane-induced lipid peroxidation in association with the anaerobic metabolism of halothane in guinea pig liver microsomes.