In single atrial and ventricular cells isolated from the guinea-pig and the rabbit heart, action potentials and membrane currents were recorded by using the whole cell voltage clamp technique. In rabbit atrial cells the repolarization showed two distinctive phases, referred as the early and late phases(early and late plateau phase), but in guinea-pig atrial cells there was a maintained plateau and less distinctive two phases of repolartization. Increasing intracellular sodium or reducing external sodium by replacement with lithium suppressed the late phase of the action potential in rabbit atrial cells and shortened the plateau of action potential in rabbit ventricle and guinea-pig atrial cells. Reducing external sodium decreased Ca-current and late inward current in voltage clamp.