A study of the ventricular activation process (V.A.P.) at the body surface of normal subjects has been carried out by means of multiple simultaneous electrocardiographic leads. In order to reduce the discrepancies between real and calculated vectors, the vectorial analysis was facilitated by means of a plastic model in which the form of the thoracic cage as well as the location of the apparent "electrical center" was particularly considered. Three main instantaneous vectors corresponding to the V.A.P. are described. The first vector ("vector 1" or "septal vector") orients always forward and, as a rule, to the right; it represents early septal electrical forces. The second vector ("vector 2i" or "left ventricular vector") points to the left, downward, and backward, toward the free left ventricular wall. It corresponds to the predominant electrical activity originated in this ventricle. The third vector ("vector 3" or "basal vector") goes, usually, backward, to the right, and upward. It represents the deplorization of basal regions of the ventricular mass, including the septum. These three vectors are inscribed, on an average, at 0.010, 0.040, and 0.064 second, respectively, after the onset of the QRS complex. The determination of the three vectors from the scalar curve permits one to follow the spatial sequence of the ventricular activation process and to infer the potential variations at different points of the body surface. © 1955.