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Ultrathin sections of blood capillaries of the myocardium of the left ventricle of Wistar rats aged from birth to 45 days were studied. The obtained data made it possible to reveal some features of the postnatal development of the endothelium and blood capillaries in the myocardium of rats. In newborn rats, blood capillaries of the myocardium are formed by light and dark dehydrated endotheliocytes. The revealed heteromorphism of the endotheliocytes of the blood capillaries of juvenile rats indicates that they are at different stages of the cell cycle. Capillary loops, in the lumen of which there are clusters of erythrocytes, are found in the MRI of the myocardium of the left ventricle of 5-day-old rats. Under the pressure of protrusions of erythrocytes, intussusceptions of the cytoplasm of the endothelium deep into the interstitium are formed. Intussusceptions lengthen and gradually transform into new blood capillaries. In the myocardium of 15-day-old rat pups, morphological manifestations of the development of physiological apoptosis of blood capillary endotheliocytes and the formation of apoptotic bodies were revealed. Removal of apoptotic bodies from the lumen of the vessels of the MCR is carried out in two ways: passively -by blood flow, and actively -with the help of erythrocytes, which transport apoptotic bodies adsorbed on the surface along the bloodstream to the sinusoidal exchange capillaries and spaces of Disse of the liver. When studying the ultrastructure of the myocardium of rats of various ages, it was found that isolated and small groups of erythrocytes are observed outside the capillaries in the connective tissue. We assume that under physiological conditions, diapedesis of erythrocytes is due to oxygen deficiency in a limited area of the myocardium. There is deoxygenation of erythrocytes and an increase in oxygen content in the local area of connective tissue. The diapedesis of erythrocytes into the connective tissue of the myocardium prevents the development of hypoxia.