Heart vector location was estimated for 23 isolated, perfused rabbit hearts durng paced ectopic beats. Twenty computer-processed signals, derived from the surface electrodes of a spherical electrolyte-filled tank containing the hearts, were optimally fitted with a locatable cardiac dipole every millisecond of the QRS and every 3rd msec of the ST-T interval. During the QRS, the computed heart vector location of hearts subepicardially paced from the left ventricular free wall originated very close to the stimulating electrode, traversed the heart from left to right, and terminated in the right ventricle. During the first portion of repolarization for the hearts paced from the left ventricle, the position of the heart vector was almost stationary within the left ventricle, whereas after the peak of the T wave, heart vector location again moved from left to right. The first quarter of the QRS interval for hearts stimulated from the right ventricular free wall was nondipolar; during the remaining three-quarters of excitation, location of the heart vector moved from right to left, terminating in the left ventricle. Throughout the entire T wave of hearts paced from the right ventricle, the position of the heart vector remained almost motionless within the left ventricle. This study demonstrates the ability of heart vector location, by its rapid motion, graphically to portray passage of an ectopic beat across the heart and, by its slower motion within the central portion of the heart, to indicate the diffuse nature of the resulting ventricular recovery.