Changes in rat cardiomyocytes and their mitochondria and intermitochondrial junctions (IMJs) upon beta-adrenoreceptor stimulation with isoproterenol were studied by the methods of light and electron microscopy and computer-aided morphometry. It was found that isoproterenol injections (0.3 mg/kg for eight days) resulted in myocardial hypertrophy, which was more pronounced in the right than in the left ventricle. In the hypertrophied cardiomyocytes of both ventricles, an adaptive response of mitochondria was observed: their ultrastructure, size, and number changed, and the number and average length of IMJs increased. A positive correlation between the degree of cell hypertrophy and the number of IMJs was revealed. The reactive properties of mitochondria, including IMJ formation, differed depending on their location in the cell (i.e., in the paranuclear, intermyofibrillar, or subsarcolemmal regions). These results suggest that the rates and intensities of adaptive compensatory processes developing in the mitochondria of cardiomyocytes exposed to beta-adrenoreceptor stimulation differ in the left and right ventricles.