Six patients undergoing surgery for ischemic heart disease were studied by means of epicardial electrodes affixed to the left ventricle. One patient who underwent triple coronary arterial bypass and plication of a left ventricular aneurysm had evidence of an acute myocardial infarction 1 day after operation. A surface electrocardiogram and a bipolar electrogram from the left ventricle were recorded before and after development of the infarction. The bipolar electrogram showed a marked loss of voltage and delay of conduction that were not present in the patients who underwent surgery but did not sustain an acute myocardial infarction. Thus, any acute myocardial infarction in conscious persons appears to be associated with areas of delayed epicardial action and diminution of local voltage. This observation suggests that the electrophysiologic mechanisms of infarction in man are similar to those of infarction in the dog.