Heart failure as one of the most common pathologies in developed countries, leading to adecrease in the quality of life, continues to remain amedico-biological problem. An important role in solving specific and general issues of cardiac pathology is given in morphological studies. Chronic heart failure (CHF) is one of the most common complications of coronarogenic and noncoronarogenic heart diseases. For the correct and unbiased interpretation of CHF, the application of some cardioprotectors for asuccessful treatment is necessary to study the structure of intact myocardium in experimental rats. Having this aim the ventricular and atrial heart of adult intact rats were investigated with routine methods of light and electron microscopy. The optical -light and electron microscopy studies have shown that myocardial fibers in the heart ventricle are more dense than in the atrium. The layer of loose connective tissue with blood vessels and nerves are between myocardial fibers. Ventricular and atrial cardiomyocytes are elongated or oval in shape, and ventricular myocytes are larger than atrial. The structure of the sarcolemma cardiomyocytes in both parts of the heart is usual. Intercalated disks «step»– «z» shape with common types of contacts are visible at the joints in contact cardiomyocytes. Nucleus is oval or round in shape with apredominance of heterochromatin and is located in the centre of the cell. Contractile apparatus is represented by typical myofibrils between which are located the mitochondria longitudinally. The mitochondria in ventricular myocytes are bigger, longer, with densely packed cristae and matrix, than in atrial myocytes. Sarcoplasmatic reticulum is developed in the ventricle myocardium and is represented by numerous channels and tubes. The elements of the Golgi complex are developed in atrial cells. There are secretory cells with numerous оsmiofilic granules in the atrial cardiomyocytes.