Current methods of experimental infarct size measurement require tissue analysis several hours after the ischemic event. Electron microscopy identifies irreversibly injured myocytes early after ischemia, but cannot be used to measure myocardial infarct size. Horseradish peroxidase (HRP), a tracer protein that permeates the disrupted sarcolemma of injured myocytes, was used to determine infarct size. New Zealand White rabbits (n = 15) were subjected to 30 min coronary artery occlusion and 24 h reperfusion. Prior to euthanasia, HRP was administered intravenously. The hearts were excised, perfused with triphenyl tetrazolium chloride (TTC), and perfusion fixed. A frozen section was cut from left ventricular slices, and the brown HRP reaction product was developed with 3,3'-diaminobenzidine tetrahydrochloride and H2O2. The areas delineated by intramyocyte HRP (as a percent of the left ventricle) closely correlated with infarct size determined by conventional hematoxylin-eosin (H&E) stain and TTC (means +/- S.E.M.): 29.5 +/- 3.0% vs 27.6 +/- 2.9% vs 28.6 +/- 2.6%, respectively. The coefficient of correlation between HRP and H&E was 0.94. This method was tested for early infarct detection in rabbits subjected to 30 min coronary occlusion followed by intravenous injection of HRP and sacrifice. HRP-delineated infarcts measured 21.4 +/- 3.7% of the left ventricle, and electron microscopic examination from ischemic and non-ischemic areas was used to confirm that cells containing HRP were irreversibly injured. Thus, HRP can be used to accurately measure myocardial infarct size in experimental coronary artery occlusion and reperfusion in infarcts as early as 30 min after coronary occlusion or following 24 h of reperfusion.