The relationship between lipid peroxidation and rat heart mitochondrial monoamine oxidase activity was studied in experimental myocardial necrosis induced by adrenaline injection. It has been established that both the intensity of peroxidation and the activity of monoamine oxidase in mitochondria from adrenaline-injured rat myocardium were essentially increased. The preliminary administration of antioxidants (vitamin E and ionol) was shown to decrease both the intensity of lipid peroxidation and the activity of monoamine oxidase. It is suggested that intensification of lipid peroxidation which is considered to be the main pathogenic factor in ischemic myocardial injury depends on mitochondrial monoamine oxidase activity. Protective effects of antioxidants are realized by the action on two subsequent chains during the formation of active oxygen forms and destruction of lipid peroxidation products.