1. Serial venous blood samples were obtained from 45 patients with acute myocardial infarction. Ten of these patients were receiving β-adrenoreceptor-blocking drugs at the time of onset of chest pain and continued on these drugs during their stay in the coronary care unit. The activities of creatine kinase and its MB-isoenzyme (CK-MB) were assayed in the plasma. A lysosomal enzyme, β- N -acetylglucosaminidase, was also assayed. 2. In the 35 untreated patients it was found that creatine kinase activity was maximal at a mean time of 21.3 ± 1.3 h after the onset of chest pain, whereas in the patients receiving β-adrenoreceptor-blocking drugs peak activity of the enzyme occurred at 24.4 ± 0.7 h. 3. Peak CK-MB activity was also delayed from 18.1 ± 1.6 h in the control group to 22.4 ± 1.2 h in the treated patients. 4. The lysosomal enzyme showed a similar pattern of changes to that of CK-MB. Maximum activity in plasma occurred at 18.0 ± 1.0 h after the onset of chest pain in the control group of patients. In the treated patients peak lysosomal enzyme activity was not found until 24.2 ± 1.2 h. 5. These alterations in the time-course of plasma enzyme changes after acute myocardial infarction are consistent with the suggestion that β-receptor antagonists may delay tissue damage during myocardial ischaemia.