The NADPH-dependent microsomal metabolism of [14C]procarbazine, labeled on the terminal N-methyl group, resulted in the covalent binding of the drug to exogenously added DNA; this reaction was inhibited by metyrapone. Procarbazine metabolism was also shown to result in covalent binding of the methyl group of the drug to microsomal protein upon metabolism, but the extent of protein binding was at least an order of magnitude smaller than that seen with its primary oxidative metabolite. N-isopropyl-alpha-(2-methylazo)-p-toluamide. The characteristics of the reactions leading to the covalent binding of the N-methyl group of the azo derivative to microsomal protein and its metabolism to form the hydrocarbon, methane, possessed a number of similarities in the apparent kinetic parameters (Km and Vmax), induction, and inhibition patterns indicating a common pathway of metabolism to form a reactive intermediate and the involvement of cytochrome P-450. Reduced glutathione stimulated methane formation and inhibited covalent binding to protein. One azoxy derivative, N-isopropyl-alpha-(2-methyl-ONN-azoxy)-p-toluamide, was chemically unstable and its decomposition was shown to lead to covalent binding to microsomal protein. A diazene intermediate and a methyl radical are proposed to be intermediates in the formation of methane during the oxidative metabolism of the azo derivative of procarbazine and a common intermediate in the activation of procarbazine may result in both covalent binding to cellular macromolecules and methane production. In addition, chemical decomposition of the azoxy metabolites may also contribute to a small portion of the covalent binding, but not to methane formation.