We reviewed the absolute amd differential growths of the myocyte populations in the left (L) and right (R) ventricular myocardium morphometrically from 1 to 5 days and from 5 to 11 days after birth. From 1 to 11 days hypertrophy of the average myocyte in the ventricles was (L) 2.7-and (R) 2.4-fold, and myocyte proliferation was (L) 2.0-and (R) 1.2-fold. Mean cell volume, cell length, and percent binucleation of cardiac myocytes were similar in both ventricles at 1, 5, and 11 days of age. During this period, average myocyte length increased 2-fold (12 sarcomere lengths), and the percentage of binucleate myocytes increased approximately from 2.7 to 17 to 48%. Myocyte hypertrophy from 1 to 5 days resulted mainly from an increase in the volume of cytoplasm per nucleus and from 5 to 11 days from the accumulation of binucleate cells. No differences were observed in the characteristics of epicardial and endocardial myocytes in either ventricle up to 11 days of age. The 61% greater proliferation of myocytes in the left ventricle was the principal basis for the development of a 2-fold difference in ventricular weight gains: (L) 6.2-and (R) 3.4-fold. No increase in right ventricular midwall thickness was observed, in contrast to a 2.7-fold increase of the left ventricle. It was concluded that, as a result of the circulatory changes occurring shortly after birth, right ventricular growth is analogous to eccentric hypertrophy, whereas left ventricular growth represents a combination of eccentric and concentric hypertrophy. Circ Res 46: 495-502, 1980 GROWTH of the myocardium during the early postnatal period must accommodate not only the increasing demands of the rapidly growing animal but also the relatively abrupt changes in the patterns of blood flow and circulatory resistance occuring shortly after birth. Work required of the left and right ventricles is approximately equal as they pump in parallel during fetal life With closure of the foramen ovale and ductus arteriosus after birth, blood flow is henceforth in series through the right and left ventricles, resulting in a greater volume work load on each ventricle. In the same period, pulmonary resistance is lowered by expansion of the collapsed lungs The combi-
Anversa et al. (1980) studied this question.