Serum aspartate aminotransferase (SGOT), alanine aminotransferase (SGPT), creatinine phosphokinase (CPK), and butyric acid dehydrogenase (BDH) were determined in 94 patients before, 1(1/2) hours, and 24 hours after cardioversion. An increase in SGOT and CPK activity was observed 24 hours after cardioversion in the group of patients treated with two or more DC shocks. The importance of this enzyme activity increase is discussed. It originates in the skeletal muscles and probably has no clinical significance, as no other signs of myocardial damage were observed simultaneously in a large group of patients.