Experiments on rats weighing 110 g exposed to sympathectomy induced by guanethidine injections during 4 weeks after the birth were made to examine the morphological characteristics of cardiomyocytes, ultrastructure of heart tissue mitochondria, heart contractility under relative physiological rest, and the pressure in the ascending aorta. It was shown that the chronic absence of the sympathetic system leads to changes in the structure of the heart tissue proved by hypertrophy of the left ventricle and disturbances of the cell mitochondrial system. Under continuous high pressure the sympathectomized rats manifest suppression of the heart contractile function. It is suggested that the changes discovered are a consequence of the disorders in the myocardial energy balance in the sympathectomized animals.