Myosin was isolated from pig atrial and ventricular myocardium during postnatal development and Ca2+-ATPase was determined and myosin light chains were analysed by electrophoresis in sodium dodecylsulfate polyacrylamide gel. During ontogenesis ATPase activity of ventricular myosin remains virtually unchanged, whereas that of atrial myosin increases. The patterns of myosin light chains of atrial and ventricular myosin differ from each other, but the individual pattern remains unchanged during the development.