The ultrastructural localization of succinate dehydrogenase in white rat heart myocytes is studied and heterogeneity of reaction products in separate mitochondria and their groups is described. The enzyme activity is cardiomyocyte electron density dependent. This dependence, in all probability, is the result of different structural and functional states of cells and their organelles, that is revealed by electron microscopy as different electron density of these. It is found that middle electron density cells have the maximum enzyme activity. The mechanisms of enzyme activity dependence of cell electron density are discussed.