Procarbazine, a 1,2-disubstituted hydrazine, is employed therapeutically in the treatment of Hodgkin's disease and a limited number of other neoplasias. The isomeric azoxy metabolites of procarbazine have recently been identified as the precursors of species responsible for both the anti-cancer efficacy and toxic effects mediated by this drug. This study demonstrates that cytosolic enzymes are involved in the metabolism of the azoxy metabolites of procarbazine. Two azoxy procarbazine oxidase activities were resolved by diethylaminoethyl (DEAE)-cellulose chromatography. The activity which did not bind to this column was purified to homogeneity and was identified as a phenobarbital-inducible form of cytosolic aldehyde dehydrogenase. This protein fraction was shown to metabolize only the azoxy 2 procarbazine isomer to yield N-isopropy-p-formylbenzamide (ALD) in a reaction which did not require NAD+ as cofactor. The ALD product formed was also a substrate for a subsequent NAD(+)-dependent reduction reaction catalyzed by that purified protein. The azoxy 2 procarbazine isomer and ALD were shown to be potent inhibitors of both the dehydrogenase and esterase activities of aldehyde dehydrogenase. The second azoxy procarbazine oxidase activity which was retained by the DEAE-cellulose column co-eluted with xanthine oxidase activity. Both the xanthine dehydrogenase/oxidase and azoxy procarbazine oxidase activities of this protein fraction were inhibited by allopurinol, a specific inhibitor of xanthine dehydrogenase. Xanthine dehydrogenase/oxidase was partially purified by an alternative procedure and was shown to metabolize both the azoxy 2 procarbazine isomer and ALD, ultimately producing N-isopropylterephthalamic acid. The ability of xanthine oxidase to metabolize azoxy 2 procarbazine and ALD was confirmed using commercial, purified milk xanthine oxidase.