The postnatal development of myocardial cells in the bilateral atria and ventricles of rats at days 1, 10, 20, 30 and 60 was studied by means of an ultrastructural quantitative method. No clear differences between the right and left myocardium were found throughout the developmental period or in the adult stage. The myocardial cells increased in thickness more rapidly up to day 20 than during the subsequent period. Myofibrils occupied about 50-60% of the cytoplasm of almost all kinds of cardiac muscle cells except those of the newborn atrium. The fraction of mitochondria increased after day 10, whereas that of glycogen conversely decreased. Although the presence of specific granules characterized the atrial myocardial cells even in newborns (day 1), no differences in other cellular components or fiber thickness between the atrium and the ventricle were seen at this stage. The distinction between atrial and ventricular muscle cells, in terms of fiber thickness, mitochondrial and glycogen content, began to appear at day 10 and became increasingly prominent with age until virtually all muscle fibers reached an almost mature state at day 20.