Several invasive and noninvasive techniques used in determining the size of experimental myocardial infarction were evaluated after acute ligation of the left anterior descending (LAD) coronary artery in ten dogs. Systemic blood pressure, left ventricular end-diastolic pressure (LVEDP), and heart rate did not change significantly for up to 24 hours after coronary occlusion. Left ventricular wall motion abnormalities were detected by left ventriculography in the distribution of the LAD but these changes did not correlate well with the infarct weight determined at autopsy. On the other hand, the number of epicardial sites with ST-segment elevation of greater than or equal to 2 mm (mean 15.1 sites +/- 0.6 SEM) and the infarct area as measured by 99mTc-glucoheptonate (TcGH) myocardial imaging (15.7 sq cm +/- 0.6) did correlate strongly with the infarct weight (16.8 g +/- 0.7) determined by the nitroblue tetrazolium (NBT) technique (r = 0.91). TcGH myocardial scintigraphy and epicardial ST-segment mapping allowed early and accurate quantification of experimental myocardial infarcts ranging from less than 1 g to 28 g.