The intracellular sodium concentration reported for young, embryonic chick hearts is extremely high and decreases progressively throughout the embryonic period, reaching a value of 43 mM immediately before hatching. This observation suggested that the ionic basis for excitation in embryonic chick heart may differ from that responsible for electrical activity of the adult organ. This hypothesis was tested by recording transmembrane resting and action potentials on hearts isolated from 6-day and 19-day chick embryos and varying the extracellular sodium and potassium concentrations. The results show that for both young and old embryonic cardiac cells the resting potential de- pends primarily on the extracellular potassium concentration and the amplitude and rate of rise of the action potential depend primarily on the extracellular sodium concentration. Evidence from many electrophysiological studies indicates that the ionic basis for the transmembrane resting and/or action potentials of adult cardiac tissues from a variety of mammalian and nonmammalian species (Overton, 1902; Draper and Weidmann, 1951; Weidmann, 1955; Weidmann, 1956; D~lez~, 1959; Brady and Woodbury, 1960; and Seyama and Irisawa, 1967) is qualita- tively similar to that described for the giant axon of Loligo (Hodgkin and Katz, 1949; Hodgkin, 1951; Hodgkin and Huxley, 1952 a, b) and the frog • eletal muscle (Nastuk and Hodgkin, 1950). Nevertheless, in several studies on cardiac fibers the magnitude of the active membrane potential (overshoot) did not vary as predicted from the sodium equilibrium potential. In both guinea pig (Coraboeuf and Otsuka, 1956) and frog ventricle (Van der Kloot and Rubin, 1962) a large positive overshoot persisted after prolonged per- fusion with solutions essentially free of Na +. More recently it was shown that tetrodotoxin, thought to cause selective inactivation of the Na-carrying system (Kao, 1966), does not abolish the overshoot of the action potential of frog ventricle (Hagiwara and Nakajima, 1965). In addition, several investigators have suggested that part of the current causing depolarization of cardiac fibers 666