A method has been developed for the determination of aldehyde oxidase. Dimethylaminocinnamaldehyde was taken as substrate. It is oxidized to the corresponding acid and changes ist absorption at 398 nm. Except dissolved oxygen no additional electron acceptor is needed. This alternative assay was found to be about twice as sensitive compared to the method with acetaldehyde and dichlorophenol indophenol. Aldehyde oxidase activity was estimated in the raw extract of livers of pig, sheep and rat. Xanthine oxidase does not interfere.