The distribution of radioactivity in the myocardium of rats and dogs infarcted by ligation of the left anterior coronary artery has been determined after intravenous injection of 9-[Te-123m]telluraheptadecanoic acid (Te-123m HDA]. In rats the normal myocardium concentrated radioactivity (3.7% +/- 0.28 injected dose/g) to nearly three times that in the zones of infarction (1.12% +/- 0.18 dose/g0. The focal defects detected in the gamma-camera images of rats and dogs correspond well with areas of infarction identified in the excised hearts by staining with triphenyltetrazolium chloride. The distribution of radioactivity from Te- 123m HDA in dog hearts sectioned at autopsy showed a linear correlation (r = 0.94) with blood flow as determined with scandium-46-labeled microspheres.