Epicardial ST-segment alterations were determined 15 min. following acute ligation of the left anterior descending coronary artery in dogs, and the extent of the left ventricular wall infarcted at each mapping site was determined 24 hours later. A close correlation was observed between the ST-segment elevation and the percent of left ventricular wall infarcted (r = .891; P less than .001) at the same mapping site. A significant portion of the left ventricular wall remained viable at sites with less than 8 mV elevation; however, the chance of transmural infarction became high at sites with elevations greater than 10 mV. It may be that sites with less than 10 mV elevation in the epicardial ST-segment elevation are ischemic areas where pharmacologic interventions designed to limit infarct size have the greatest change of success.