Mitochondrial cytochrome c oxidase is able to oxidize various aromatic compounds like o-dianisidine, benzidine and its derivatives (diaminobenzidine, etc.), p-phenylenediamine, as well as amidopyrine, melatonin, and some other pharmacologically and physiologically active substances via the peroxidase, but not the oxidase mechanism. Although specific peroxidase activity of cytochrome c oxidase is low compared with classical peroxidases, its activity may be of physiological or pathophysiological significance due to the presence of rather high concentrations of this enzyme in all tissues, as well as specific localization of the enzyme in the mitochondrial membrane favoring accumulation of hydrophobic aromatic substances.