The authors assessed the time course of the S-T segment in 12 standard and monitor ECG leads and the time course of MB CPK fraction activity and Mg blood concentration in 34 patients with acute myocardial infarction. After the detection of thrombotic occlusion of the coronary artery they received thrombolytic therapy with angiographic control. Seventeen patients with recovered coronary blood flow demonstrated a considerable decrease in the index of total shift of the S-T segment in leads I, II, III (in inferior MI) and in V1-V6 (in anterior MI) by the 3rd hour of treatment, a decrease in the S-T segment over 50% by the 3rd hour in one of 12 standard leads, a rise of a segment in which basal S-T was maximum, a rapid time course of the S-T segment in monitor ECG leads and early peaks of MB CPK activity and Mg blood concentration. The results obtained, an analysis of the time course of the S-T segment, and assessment of the time of attainment of maximum MB CPK activity and Mg blood concentration make it possible to judge, with a high degree of probability, of the occurrence of coronary reperfusion in MI patients.