During myocardial ischemia produced in 43 dogs by occlusion of left coronary artery, electrical alternans developed in 34 experiments. The most common was alternans of ST-T complex. Surface and intracellular electrograms were recorded simultaneously from contiguous sites in the ischemic area. The alternans of ST-T complex in the surface electrogram corresponded to that of the rate of repolarization of the membrane action potential. The development of this alternans is localized in 2 relatively small area and transient. This may be the reason why electrical alternans of ST-T is clinically rare. A clinical case showing electrocardiographic changes of electrical alternans of ST-T complex without any change in the QRS complex is reported. Myocardial infarction and hypokalemia is considered as a cause of the alternans. The present results support the hypothesis that the mechanism of electrical alternans will be an alternation of the rate and extent of the ions transported across the myocardial cell membrane.