An electrophysiologic investigation of the normal and ischemic left ventricle is conducted using the chronic conscious pig as the experimental model. The experimental conditions imposed upon this preparation include mechanical occlusion of the left anterior descending coronary artery, cerebral stimulation and administration of an anti-arrhythmic drug, propranolol. The occlusion experiments are primarily of a temporary nature in order to study the early effects of acute myocardial ischemia; long term occlusion experiments are also conducted in order to investigate the transition from severe ischemia to myocardial infarction. The central nervous system is postulated to exert a major influence in the electrogenesis of cardiac arrhythmias and ventricular fibrillation in the ischemic heart. Psychological adaptation of the conscious animal to the experimental environment appears to prevent ventricular fibrillation following coronary occlusion. Administration of propranolol in sufficient quantity to block the betaadrenergic nervous receptors reinstates fibrillation. It is concluded that the sympathetic division of the autonomic nervous system normally projects a beneficial activity to the myocardium; however, no sympathetic innervation (as in beta-receptor blockade) or increased sympathetic activity (as in the anxious or excited animal) have a detrimental effect on the stability of the ischemic heart, and ventricular fibrillation occurs. The electrophysiologic changes which occur during the first twenty minutes (the period of reversible injury) of ischemia provide interesting results. Rapid alterations seem to occur approximately during the second minute and the tenth minute of ischemia, which correspond to periods of excessive instability of the heart. This is most evident in the increased amplitude of bipolar potentials from the outer layers of the ischemic ventricular wall and the larger S-T segment elevations. The importance of these findings with regard to the almost national epidemic of coronary heart disease and sudden death is discussed.