Twenty-nine patients were randomly allocated to two groups before undergoing aorto-coronary bypass surgery. In one group the beta-blocking medication was withdrawn three days preoperatively, and in the other group it was maintained. The patients in the latter group were additionally given 100 mg metoprolol per os two hours before surgery. The degree of myocardial injury, as judged from cumulated activity of S-CK B, was less when the beta-blockade was maintained.