Because the form of QRS from the body surface of pigs is different from that of carnivores or ungulates, and because that form is dependent upon pathways of ventricular activation, this study was designed to study pathways of ven tricular activation in pigs. Twelve pigs were anesthetized and right or left hemithoracotomies were performed to expose the heart. Contiguous bipolar electrograms were recorded from button electrodes on the epicardium and from both faces of the interventicular septum, and from multipolar plunge electrodes introduced into the intramural regions of both ventricles. Electrograms were recorded simultaneous with the Z-axis ECG at 625 mm/sec paper speed on a photographic oscillograph. Times of arrival of waves of activation at numerous points in the ventricle were referenced to the peak of the R-wave in the Z-axis ECG. During the initial 10 msec of QRS, the apical-third of the interventricular septum is activated from left to right. During the next 40 msec of QRS, waves of activation originating at the cranial portion of the right ventricle and the caudal portion of the left ventricle engulf the epicardium toward the interventricular septum and slightly in an apico-basilar direction. Activity begins slightly earlier at the caudal aspect of the left ventricular free-wall and terminates on the pulmonary conus region. During the terminal 30 msec of QRS, the basilar third of the interventricular septum is activated in a general apico-basilar direction. Through regions of either right or left ventricular free-walls was a general endocardial to epicardial activation observed. These pathways of ventricular activation may be explained by the rather complete penetration of Purkinje fibers through both ventricular free-walls in a manner similar to that of ungulates but different from carnivores and primates.