The metabolism of isaxonine was first investigated in mice. Incubation, under air, of [2-14C] isaxonine (1 mM) with mouse liver microsomes and an NADPH-generating system resulted in the irreversible binding of a [14C] isaxonine metabolite to microsomal proteins; binding required active microsomes, NADPH and oxygen, it was inhibited by 4 mM piperonyl butoxide or by a CO-O2 (80:20) atmosphere. In the presence of various concentrations of isaxonine (0.125-2 mM), binding followed Michaelis-Menten kinetics; the Vmax was increased by both phenobarbital and 3-methylcholanthrene pretreatments. In vivo, 2.5 hr after the administration of [2-14C] isaxonine (4 mmol X kg-1 i.p.), a [14C] isaxonine material was irreversibly bound to mouse liver proteins; this binding was decreased by piperonyl butoxide and increased by phenobarbital or 3-methylcholanthrene pretreatments. Irreversible binding also occurred in the kidney. Unlike their effects in the liver, piperonyl butoxide and phenobarbital did not modify significantly in vitro metabolic activation by kidney microsomes and in vivo covalent binding to kidney proteins; pretreatment with 3-methylcholanthrene increased both in vitro and in vivo binding in the kidney. In a second series of experiments, in vitro metabolic activation was demonstrated with human liver microsomes; as in mice, covalent binding required NADPH and was markedly inhibited by piperonyl butoxide. We conclude that isaxonine is activated by mouse and human cytochromes P-450 into a reactive metabolite. In vivo covalent binding to mouse liver and kidney proteins appears to result mainly from the in situ binding of the metabolite formed in each organ.