Backgroud. The dynamics of morphogenetic rearrangements of myocardial mitochondriom under the influence of hypothyroidism and electromagnetic radiation remains insufficiently studied. These circumstances determine the relevance and need to study the mitochondrial reactions of cardiomyocytes in these conditions. Objective.Determination of the reactions of the contractile cardiomyocytes mitochondrial apparatus of the left ventricle of rat heart under hypothyroidism on the effect of various exposures to electromagnetic radiation. Methods. We studied samples of the intramural zone of the free wall of left ventricle in sexually mature rats. Experimental animals were divided into 6 groups: 1) control (intact rats) 2) rats with hypothyroidism; 3) rats after general electromagnetic irradiation with an exposure of 45 minutes 4) rats after irradiation with an exposure of 120 minutes 5) rats with hypothyroidism after irradiation with an exposure of 45 minutes 6) rats with hypothyroidism after irradiation with an exposure of 120 minutes. The hypothyroid state was modeled using thyroidectomy. The effect of electromagnetic radiation was carried out daily for 10 days on the whole body. Ultrastructural analysis of the mitochondrial apparatus was performed using transmission electron microscopy. Results. Experimental hypothyroidism in rats was accompanied by an increase in mitochondrial heteromorphism of contractile cardiomyocytes of the left ventricle, cristolysis of myofibrillar mitochondria, and the appearance of giant organelles with severe swelling of the matrix. After irradiation with an exposure of 45 minutes, the overall structure of the mitochondrial apparatus is unchanged. An exposure of 120 minutes caused the destruction of subsarcolemmal and paranuclear organelles, edema and degradation of intermyofibrillar mitochondria. After exposure for 45 minutes under conditions of a hypothyroid state, mitochondria in paranuclear localization were represented by enlarged organelles with a vacuolar type of damage, without signs of destruction of the external membranes, moderate cristolysis, with an electronically clear matrix. Exposure for 120 minutes in animals with hypothyroidism led to edema and destruction of most mitochondria, destruction of cristae, effects of excessive tension on preserved organelles, damage to the structure of intermitochondrial contacts, and inhibition of reproduction of mitochondria. Conclusion. After exposure to electromagnetic radiation for 45 minutes under hypothyroid conditions, there is a compensatory restructuring of the energy apparatus of contractile cardiomyocytes of the left ventricle due to neoplasm of mitochondria and an increase in their contact interaction. After the action of irradiation for 120 minutes, the development of destructive-degenerative processes in the mitochondrial apparatus of left ventricular cardiomyocytes is observed, and damage to intermitochondrial contacts is extended.