Pig liver and pig brain mitochondrial monoamine oxidase were inhibited by increasing concentrations of clorgyline (selective inhibitor for the "A" form of monoamine oxidase) and deprenil (selective inhibitor for the "B" form of the enzyme) and the activities were then estimated with serotonin, tyramine and beta-phenylethylamine as substrates. The results indicate that both the "A" and the "B" forms are present in these tissues. Serotonin and tyramine are shown to be oxidized by both the "A" and the "B" forms of the enzyme, whereas beta-phenylethylamine appears to be oxidized almost exclusively by the "B" form. Lipid-depletion of the mitochondrial preparation from these tissues by extraction with aqueous methyl ethyl ketone eliminated almost all of the "A" form activity while most of the "B" form activity remained.