Myoglobin, total CK and MB CK isoenzyme were determined in the sera of 6 patients admitted with arrhythmias and treated with D.C. countershock and also in 37 patients with acute myocardial infarction or anginal syndrome. All the three tests were increased in patients with myocardial infarction. Serum myoglobin seems sufficiently sensitive and specific but the time required for the assay is a limiting factor for practical use in emergencies. After electrical cardioversion, myoglobin and CPK remain normal, although MB isoenzyme was increased in two of six patients. These results suggest that electrical cardioversion in itself does not influence serum levels of myoglobin. The occasional increase of MB CK observed after cardioversion seems to be the consequence of an easier release of the enzyme due to a myocardial injury.