Both aldehyde oxidase and xanthine oxidase catalyze the oxidation of a wide range of N-heterocycles and aldehydes. These enzymes are important in the oxidation of N-heterocyclic xenobiotics, whereas their role in the oxidation of xenobiotic aldehydes is usually ignored. p ]The present investigation describes the interaction of methyl- and nitrosubstituted benzaldehydes, in theortho-,meta- andpara-positions, with guinea pig liver aldehyde oxidase and bovine milk xanthine oxidase.