The response of cardiomyocytes and the mitochondrial system of rats subjected to a four-month hypokinesia was studied. Morphometric light microscopic analysis of cardiomyocytes of the left ventricle revealed an increase of the area of cell cross sections, whereas the number of capillaries remained the same in the normal heart, which was suggested to be a weak adaptive response. In electron microscopy, along with normal cardiomyocytes, cells with the swollen sarcoplasm and degrading myofibers were observed, which indicated a partial atrophy of the heart muscle. Heterogeneous ultrastructural changes of mitochondria were also observed. Some organelles were swollen. Mitochondria with unusual dark cristae and mitochondria with cristae in a "twisted" configuration were found. The number of intermitochondrial junctions considerably decreased in all regions of mitochondrial location which is presumably related to the decrease in the functional cell activity under experimental hypokinesia.