The relationship between regional blood flow and epicardial S-T segment elevation was studied in 26 open-chest anesthetized dogs with left anterior coronary artery ligations. Changes in myocardial blood flow, measured with 15 plus or minus 5mu (diameter) microspheres labeled with 141-Ce, 85-Sr, and 169-Yb, were correlated with summated S-T segment elevations 15 minutes, 1 hour, and 2 hours after coronary artery occlusion. In normal areas, myocardial blood flow was 113 plus or minus 5 ml/min 100 g- minus 1 and summated S-T segment elevation was 0.3 plus or minus 0.2 mv. Fifteen minutes after coronary artery occlusion in 26 dogs, S-T segment elevation was 5.7 plus or minus 0.7 mv over the center of the infarct and myocardial blood flow was 10 plus or minus 1 ml/min 100 g- minus 1; over the border zone, myocardial blood flow was 63 plus or minus 4 ml/min 100 g- minus 1 and S-T segment elevation was 3.1 plus or minus 0.1 mv. One third of the areas with a myocardial blood flow of 10 ml/min 100 g- minus 1 or less had no S-T segment elevation. In the center and border zones of the infarct in 9 dogs, myocardial blood flow increased from 11 plus or minus 2 and 67 plus or minus 8 ml/min 100 g- minus 1 15 minutes after occlusion to 20 plus or minus 4 and 84 plus or minus 12 ml/min 100 g- minus 1, respectively, 2 hours after coronary artery occlusion. These increases were not associated with a significant reduction in summated S-T segment elevation. The results do not suggest a simple quantitative relationship between epicardial S-T segment elevation and myocardial blood flow following acute coronary artery occlusion.