The authors have investigated the morphological and enzyme cytochemical differences at the level of electron microscopy in atrial and ventricular myocardium of rat and guinea pig. The ultrastructural and morphometrical observations have revealed that atrial cells tend to have very few T tubules and that Golgi complex and specific dense granules were well developed in atria compared to ventricles. Histochemically, the reaction product of Na-K ATPase was localized on the plasma membrane and T-tubules in rats similar to that of guinea pigs. The DAB positive bodies were localized in proximity to mitochondria and sarcoplasmic reticulum and measured from 0.1μ to 0.5 μ in diameter in both animal species and were noted both at pH 5.0 and pH 9.0 in rat myocardial cell. However, in the guinea pig the reaction was observed only at pH 9.0 or very faintly at pH 5.0. The DAB positive bodies were localized in subsarcolemmal region, close to the sarcoplasmic reticulum in atria. The activity of acid phosphatase in atria was higher than ventricles since the Golgi complex and subsarcolemmal citern were more highly developed in atria, but the specific granules in atria were devoid of the acid phosphatase activity. Alkaline phosphatase was confined to the endothelium of blood vessels and on the plasma membrane of heart cells. The glucose-6-phosphatase was distributed in the sarcoplasmic reticulum, the nuclear membrane and the Golgi complex.