Myocardial ischemic injury was created acutely in pigs by a closed-chest technique utilizing an intracoronary balloon occluder for the interruption of flow in the left anterior descending coronary artery and ST-segment elevation was followed over a two hour period using an 18 lead precordial map. In an experimental group of 10 animals, occlusion was carried out within the left anterior descending coronary artery 8.3 plus or minus 0.5 cm distal to the origin of the main left coronary. Mean ST segment elevation (ST) showed a peak rise of 0.16 mV 10 minutes after occlusion. The balloon was moved proximally 1.6 plus or minus 0.2 cm giving a significant secondary rise of 0.16 mV within 5 minutes, despite indications of a generally small area of additional myocardial involvement, as judged from anatomic distribution of additional vessels occluded as well as a lack of significant change in hemodynamic parameters. In a control group of 5 additional pigs, a single distal occlusion at 6.4 plus or minus 0.9 cm from the origin of the main left coronary was produced by an identical technique. The ST rose to a peak of 0.20 mV at 15 minutes and was followed by a steady decline. Unlike the experimental group, no additonal rise in ST was seen. The technique of precordial mapping thus appears to be a sensitive index of myocardial injury. In addition, it appears from this study that the magnitude of ST elevation is a direct reflection of the extent of myocardial injury.