The activity of hepatic aldehyde oxidase from rabbit, guinea pig, rat, marmoset, dog, baboon and man was investigated in vitro with charged and uncharged N-heterocyclic substrates: Km and Vmax values were determined for phthalazine, 6,7-dimethoxy-1-[-4-(ethylcarbamoyloxy)piperidino]phthalazine (carbazeran), quinine and quinidine. The oxidation of N-phenylquinolinium chloride to N-phenyl-2-quinolone and N-phenyl-4-quinolone was followed spectrophotometrically. Rat or dog liver showed low and negligible enzyme activity respectively, whereas baboon liver contained a highly active aldehyde oxidase. Enzyme from marmoset and guinea pig liver had the closest spectrum of activity to human liver aldehyde oxidase. Unlike that from man, rabbit hepatic aldehyde oxidase was refractory towards carbazeran and converted N-phenylquinolinium chloride predominantly to the 2-quinolone. N-Phenyl-4-quinolone was the major oxidation product with enzyme from guinea pig, marmoset, baboon and man.