The effects of verapamil on epicardial ST segment elevation, regional myocardial metabolism and collateral blood flow were studied in open-chest anesthetized dogs following left anterior descending coronary artery occlusion. Collateral blood flow was measured by radioactive microspheres (15 +/- 5 micron diameter) and regional metabolism was studied by measuring lactate concentration in venous blood draining the infarcting myocardium. Verapamil (0-2 mg/kg intravenously) produced a significant reduction (50-60%) in the epicardial ST elevation when it was given before coronary occlusion; when administered 15 minutes after coronary occlusion and infusion continued for two hours, it minimized (30-40%) ST segment elevation, and prevented the fall in cardiac index and rise in systemic resistance found in the untreated animals in which the ST segment remained persistently elevated. Changes in epicardial ST segment occurred without alterations in the QRS duration. Verapamil had no effect on either the total collateral blood flow or the relative distribution of flow to the endocardial and epicardial halves of the ischemic ventricular myocardium. No significant differences were found between the levels of lactate in blood sampled from small epicardial veins at the center of the infarct when the control animals were compared with those treated with verapamil.