Even at concentration as high as 20 m m, thioacetamide neither results in any type of spectral change by interaction with liver microsomal suspensions nor modifies the in vitro NADPH cytochrome P-450 reductase activity. Thioacetamide administration to rats does not induce a microsomal lipid peroxidation process. The ability of thioacetamide to induce liver necrosis in rats is age dependent, appearing at about the age of 20 days; the 30-day-old rats are already fully responsive to thioacetamide action. Thioacetamide-induced necrosis is as intense in control males as is in females or in castrated males. The previous administration of inhibitors of cytochrome P-450-mediated transformations does not prevent thioacetamide-induced necrosis (2-diethylaminoethyl-2,2-diphenylvalerate hydrochloride, SKF 525A, or ethyl N-2-diethylaminoethyl-2-phenyl-2-ethylmalonate, Sch 5706). The effect of thioacetamide on livers from phenobarbital-preinduced rats is as intense as it is in control rats. Previous treatment with either cystamine or disulfiram partially prevented thioacetamide-induced liver necrosis. The results suggest that cytochrome P-450 or NADPH cytochrome P-450 reductase does not control the process of activation of thioacetamide to the ultimate necrogen.