Timed pregnancies were obtained in Sprague-Dawley rats, and cardiac tissues from embryos of days 10, 11, 12, 13, 14 and from newborn rats were used for the cytochemical localization of ATPase activity utilizing a lead phosphate precipitation procedure. Following incubation with ATP as the substrate, granular deposits of reaction product are discernible on the cell membranes of the embryonic myocardium. There is a noticeable decrease in the intensity of reaction product as visualized in the electron micrographs from the 10th day of gestation to the 14th day. No granular reaction product is recognizable in myofibrils, mitochondria or other organelles in the cytoplasm. It appears that there is a selective deposition of the reaction product on the cell membranes or structures derived from it. The intense ATPase activity seen on 10th and 11th days seems to be correlated with the initial appearance of myofilaments and fibrils in the myocardial cells.