The results of electronmicroscopic study of the right atrium in experimental hypercholesterolemia revealed a disturbance of ultrastructural organisation of both cardiomyocytes and endothelial cells of blood microvessels. Intake of L-arginine--a precursor of synthesis of nitric oxide (NO)--with atherogenic diet was found to prevent to some extent the damage of ultrastructural organisation of cardiomyocytes and to stimulate the production of atrial granules. A significant number of secretory components, which differ in terms of structure and function, revealed in the endothelium of blood microvessels may testify to the fact that upon administration of L-arginine the paracrine effect of hormonal regulation is enhanced manifesting itself through an increase of synthesis and secretion of natriuretic hormone in atrial cardiomyocytes. A tissue hormonal system is displayed through an increase of synthesis and secretion of atrial natriuretic hormone in cardiomyocytes. On the other hand, a NO precursor L-arginine may produce a correcting effect upon the ultrastructure of endotheliocytes in hypercholesterolemia.