It has already been shown that 3-amino-1,2,4-triazole inhibits the biological transformation of halothane in liver. The results of this study indicate that inhibition of the transformation of halothane prevents some of the changes in liver metabolism observed after halothane treatment. Changes in enzyme activities were measured as sensitive indicators. Halothane and its stable metabolite trifluoroacetate greatly increased the activity of the mitochondrial glycerol-3-phosphate oxidase. Pretreatment of rats with aminotriazole inhibited the activity increase caused by halothane, but not by trifluoroacetate. Aminotriazole also prevented the increase in the activity of malic enzyme. NADPH-oxidase, and lactate dehydrogenase produced by halothane treatment. The induction of glycerol-3-phosphate oxidase could not be counteracted by actinomycin D because actinomycin itself increased the activity of glycerol-3-phosphate oxidase.