The possible myocardial injury induced by electrical cardioversion in the treatment of supraventricular arrhythmias was analyzed after 45 countershocks of 200 joules and after 6 of 600 joules (total accumulated energy). In all patients myocardial injury was evaluated by serial CK, CK-Mb, and myoglobin determinations and in 48 cases a cardiac gammagraphy with technetium pyrophosphate was also performed. Significant increases in total CK and myoglobin were observed in 2 (4%) patients treated with 200 joules and in 3 (50%) patients receiving 600 joules. CK-Mb was also increased in one patient receiving 200 joules (13.5 U/I) and in one patient treated with 600 joules (27.8 U/I). In all cases technetium gammagraphy was negative.