The development of alimentary hypercholesterolemia in rabbits (confirmed by morphometric, electrophysiological and biochemical data) was accompanied by a decrease of the serotonin, benzylamine and tyramine deamination rates in heart muscle mitochondria. At the same time a qualitatively new reaction of cadaverine deamination could be seen in the mitochondria. The data obtained suggest that impairment of deamination of the nitrogenous compounds in atherosclerosis may be due to reversible qualitative modification (transformation) of mitochondrial monoamine oxidase activity. Some of the drugs which decrease the level of lipids in blood serum of hypercholesterolemic rabbits abolished and prevented the impairment of deamination of nitrogenous compounds.