Electrical cardioversion is reported to be associated with some degree of skeletal muscle and myocardial damage. In the present study, total creatine kinase (S-CK) and the activity of the subunit S-CK-B activity have been measured in serum after elective cardioversion in 30 patients. The enzyme activity peaked during a 28-h observation period. S-CK increased from 72 +/- 6 (mean +/- SEM) Ul-1 (1.2 +/- 0.1 ukat l-1) to a maximal value of 990 +/- 258 u l-1 (16.5 +/- 4.3 ukat l-1) and S-CK-B (analysed as a measure of S-CK-MB) increased from 3.0 +/- 0.6 U l-1 (0.05 +/- 0.01 ukat l-1) to a maximum of 10.2 +/- 1.8 U l-1 (0.17 +/- 0.03 ukat l-1), with seven patients reaching a S-CK-B value above the discrimination limit for myocardial infarction. The relationship between S-CK and S-CK-B values, however, indicated a skeletal muscle origin of the enzyme release. The maximal activity of both S-CK (r = 0.79; P less than 0.001) and S-CK-B (r = 0.70; P less than 0.001) correlated positively to the cumulative delivered energy. Thus, the release of S-CK and S-CK-B after elective cardioversion is correlated to the cumulative energy delivered, indicating increased skeletal muscle damage with greater energy. If the S-CK activity curve is interpreted without access to the S-CK-B activity this might interfere with the diagnosis of myocardial infarction.