The absolute and differential growths of the capillary network and of myocyte cytoplasmic components in the left (L) and right (R) ventricular free walls were measured morphometrically from 1 to 5 days and from 5 to 11 days after birth. From 1 to 11 days, capillary length, luminal volume, luminal surface area, and endothelial cell volume each increased 2-3 times more rapidly than myocardial mass or myocyte mass in each ventricle. Mean intercapillary distance and the transverse crosssectional area of the average capillary decreased markedly. The mean number of capillaries across the ventricular walls increased from 16 to 79 (L) and 14 to 22 (R). Maturation of the cytoplasm of left and right ventricular myocytes from 1 to 11 days included increases in the volume percentage of myofibrils(1.2-fold), mitochondria (1.8-fold), and smooth endoplasmic reticulum (2.1-fold) and increases in mean mitochondrial size [1.9-fold (L), 1.2-fold (R)] and number per cell [2.7-fold (L), 3.6-fold (R)]. Despite a 2-fold greater overall left ventricular growth, the myocardial compositions of both ventricles were nearly indistinguishable at 1 and 11 days. Both subcellular and microvascular changes, however, were generally achieved more rapidly in the left ventricle from 1 to 5 days of age, demonstrating many structural differences and a lagging development in the right ventricle at 5 days. Thus, postnatal myocardial adaptation to the altered work demands on the left and right ventricles shortly after birth resulted in morphological changes that could be the basis for a transient disparity in ventricular functions at about 5 days of age.Circ Res 46: 503-512, 19S0