Light microscopic histochemical studies to visualize semicarbazide-sensitive and H2O2-generating amine oxidase (SSAOX, benzylamine oxidase, BAOX; EC 1.4.3.6?) are usually performed with the coupled peroxidatic oxidation technique of Ryder et al. [25]. For methodological reasons this procedure has its limitations and was therefore replaced by a more reliable and easier to perform cerium-DAB-H2O2-Co technique. With this method SSAOX was studied in many organs of various laboratory rodents and marmosets and in human placenta. Independent of the species the enzyme was present mostly in the plasma membrane of nearly all vascular and non-vascular smooth muscle cells. However, there was a species-dependence of SSOX activity; the highest amounts of stain were found in gerbils and marmosets. In addition, the enzyme was found in these two species in the capillary endothelium of some extra-nervous tissues. The plasma membrane localization of SSAOX and plasma membrane-associated H2O2 production suggest a functional role for the enzyme different from that of other amine oxidases which appear to be primarily involved in intracellular amine detoxification or degradation.