Retinal oxidase (EC 1.2.3.11) and aldehyde oxidase (EC 1.2.3.1) were compared with respect to their enzymatic and physiochemical properties. It was found that the molecular weights, subunits, optical spectra, chemical and immunological properties, cellular localization and substrate specificities of the two oxidases are identical on almost all points. The physiological function of aldehyde oxidase appears to be the synthesis of retinoic acid from retinal.