Standard, augmented limb leads and lead V(10) (representing the Z axis) taken in sequence and three semi-orthogonal leads (I, aVF, and V(10)) taken simultaneously were recorded from 43 healthy pigs. Records were analyzed for rate, rhythm, interval duration and component amplitudes. The wave form of QRS complexes were analyzed in all leads studied. P, QRS, and T vectors were calculated for the mean dorsal (frontal), sagittal, and transverse planes. study of orthogonal leads in right sternal recumbency indicated that ventricular activation is spatially oriented dorsad, sinistrad and slightly caudad. No essential difference was noticed in the conventional and miniature swine. Ventricular fibrillation induced electrically in four older pigs was a progressive, terminal event; in two newborn piglets, ventricular fibrillation induced several times was terminated in each case by spontaneous recovery to sinus rhythm.