With tyramine as substrate, a considerable part of the amine oxidase activity of rat aorta was inhibited by 0.1 mM semicarbazide. The residual activity was little affected by 1 mM semicarbazide. Oxidation of 5-hydroxytryptamine was not inhibited by 0.1 mM semicarbazide. The subcellular location of the semicarbazide-sensitive and semicarbazide-resistant amine oxidases was investigated by analytical density gradient centrifugation. The semicarbazide-resistant enzyme was identified with the mitochondrial monoamine oxidase, located in the outer envelope of mitochondria. The semicarbazide-sensitive amine oxidase was ascribed to the plasma membrane because it was distributed like 5'-nucleotidase and (oligomycin-insensitive) Mg2+-ATPase in various fractionation experiments, and markedly shifted by digitonin towards higher equilibrium densities in sucrose gradient.