Biogenic amines (phenylethylamine, tyramine, dopamine, tryptamine, serotonin and spermine) decreased activities of the rotenone-insensitive NADH-cytochrome c reductase, the succinate cytochrome c reductase and the succinate dehydrogenase in incubation mixtures containing mitochondrial membranes and the monoamine oxidase inhibitors chlorgyline and deprenyl. Spermine exhibited its influences at lower concentrations as compared with other monoamines. The effect of monoamines depended on presence of hydroxyl group (or groups) in the aromatic ring which led to elevation of the inhibitory effects. Lysis of membranes by Triton X-100 prevented the inhibitory action of all the compounds studied on succinate dehydrogenase. Alterations in content of some biogenic amines under pathological conditions and especially in drug inhibition of the monoamine oxidases appear to influence the energy functions of mitochondria, possibly via changes in the surface charge of the membranes.