Bovine serum amine oxidase is inhibited by benzylhydrazine (BHy), but recovers full activity after a few hours incubation [Hucko-Haas & Reed (1970) Biochem. Biophys. Res. Commun. 38, 396-400]. The first phase of the process, requiring about 15 min, was found to consist of a mechanism-based hydrazine-transfer reaction leading to formation of the hydrazine-bound enzyme, benzaldehyde and H2O2. At variance with the enzymic process, the reaction with O2 preceded the benzaldehyde release. Two reaction intermediates could be characterized by optical spectroscopy and were assigned as the azo derivative and the benzaldehyde hydrazone, the latter one probably being involved in the reaction with O2. No reduction of Cu was detected at any stage. The hydrazine adduct could also be obtained by stoichiometric reaction of hydrazine with the native enzyme. The decay of this species occurred in about 8 h and was not studied in detail. The Cu-binding inhibitor NN-diethyldithiocarbamate affected the BHy reaction by stabilizing the benzaldehyde hydrazone form as against the azo derivative and the reaction with O2. However, under these same conditions the initial spectroscopic properties of the diethyldithiocarbamate adduct were recovered if the oxidase was left overnight. The reaction with O2 was abolished only upon removal of at least one Cu atom from the enzyme. On the basis of the failure to detect any change of Cu redox state and the enzyme behaviour in the presence of inhibitors, a reaction mechanism involving the formation of a hydroperoxy intermediate, as in the FAD-containing enzymes, is tentatively proposed.