Due to the fact that there were difficulties in interpreting the cardiac scintigrams after 99mTC pyrophosphate had been given to patients with coronary heart disease without acute myocardial infarction, an experimental study was undertaken. The scintigraphic characteristics were examined in 10 cats following ligation of the interventricular artery at its middle third for more than 20 min, followed by myocardial reperfusion, histochemical and electron microscopic studies. Cat interventricular artery occlusion for a more than 20 min was found to be followed by specific ischemic changes in ECG and myocardial accumulation of 99mTc pyrophosphate. The histochemical and electron microscopic studies indicated that there were both reversible and irreversible cardiomyocyte lesions. Reversible myocardial changes were detected not only in the ischemic area, but in the other myocardial regions away from the basin of the ligated artery. If occlusion was short, the rate of myocardial tracer accumulation rapidly became lower; with long-term occlusion or profound myocardial damage caused by reperfusion, tracer accumulation became higher. There is experimental evidence for applying 99mTc-pyrophosphate scintigraphy in the clinical setting to reveal reversible myocardial changes that are most common in chronic coronary heart disease.