Ventricular intramural and epicardial potential distributions were measured during normal excitation and repolarization in intact dogs. Potential distributions were chosen because they can be unambiguously measured, are useful in understanding the shapes of wave forms at many specific sites, and provide a direct measure of repolarization. Unipolar wave forms were recorded from intramural and epicardial electrodes and converted into potential distributions. Well-known shapes of wave forms recorded at the inner and outer layers of the ventricles as well as peak-to-peak voltages were shown by the potential distributions to be determined primarily by superposition effects of distant excitation waves. These effects were most prominent before epicardial breakthrough and then receded during the last half of the QRS complex. However, the potential distributions became more complex as excitation waves merged, collided, and terminated. During terminal depolarization, there were scattered positive repolarization potentials intramurally. Normal repolarization was characterized by positive potentials over the ventricular epicardium while there were changes intramurally and on the atrium. Throughout the T wave, there was a predominant transmural unidirectional gradient with the inner wall being more negative than the outer wall. This finding confirms that the sequence of repolarization is from the epicardium to the endocardium with the middle layers having an intermediate time.