Changes in regional blood flow in myocardial infarction experimentally induced in adult dogs have been assessed before and after treatment with normal saline, heparin and a thrombolytic agent. The technic used to measure the local blood flow and its distribution is based on the sequential injection of radioactive microspheres labeled with different isotopes. The resulting data showed that an average of 2.6% of the dose given is distributed in the normal left ventricular wall. Following infarct formation, this value drops to a mean of 0.45%. No appreciable increase was noticed after six hours of normal saline administration. After six hours of heparin infusion, the regional blood flow doubled, provided that the time elapsed between obstruction and the beginning of treatment did not exceed three hours. Fibrinolytic treatment resulted in a remarkable increase of regional blood flow (up to three times that of the postinfarction state). The shorter the time between the beginning of obstruction and commencement of treatment the better the restoration in the regional blood flow.