Experimental infarctions were produced in 12 dogs by ligation of the Ramus interventricularis anterior of the left coronary artery, and their uptake of 99mTc-heparin was measured 7, 24 and 48 h as well as 7 days thereafter by recording an in vivo chest scintigram 4 h after intravenous injection of the compound. The dogs were subsequently killed, the heart removed and an in vitro scintigram obtained. Finally, the uptake of the compound was measured in tissue samples. The results indicate that 99mTc-heparin is accumulated in lesions which are not older than 24 h. At the other points in time uptake is low but even 24-h old lesions cannot be demonstrated reliably. The results are impaired by the high blood activity on the cardiac chambers and by the background activity of the lungs. In general, 99mTc-heparin has less useful properties than the more frequently employed 99mTc-pyrophosphate so that a change of procedure is not to be recommended.