The configuration, in leads I and III of the electrocardiogram, of ventricular extrasystoles elicited by stimulation of points on the endocardium and the immediately opposite epicardium was studied in 13 dogs. The direction of initial deflections of endocardial and epicardial extrasystoles was the same in both leads I and III over most of the surface of the heart. These regions included: the area over the septum, both anterior and posterior; an area on the lateral margin of each ventricle, which is referred to as the center of the ventricle because it is equidistant from the septal margins; and the entire anterior surface of the right ventricle. The direction of initial deflections in lead I of the epicardial and endocardial extrasystoles was also found to be the same over the entire posterior portion of the right ventricle and the anterior part of the left ventricle. The direction of initial deflections in lead III of epicardial and endocardial extrasystoles was the same over much of the posterior portion of the right ventricle and the anterior part of the left ventricle. Two areas were found, the anterior left ventricle toward the left lateral border and the posterior right ventricle toward the right lateral border, where the initial deflections of endocardial extrasystoles were opposite to those of extrasystoles from immediately overlying epicardial points. These oppositely directed initial complexes of extrasystoles elicited from such regions of the endocardium were shown to arise, as do the similar complexes from stimulation of near-by epicardial points, from excitation of the opposite ventricle.