Acute ligation of the left coronary artery of rats produced abnormal Q-wave in the electrocardiogram, tachycardia and frequent ventricular fibrillation. Serum CPK level was elevated, reaching a maximum at 3 to 5 hours after ligation and returning to near the pre-ligation level 24 hours later, when CPK activity in the left ventricle markedly decreased. Pretreatment with bucumolol at 2.5 mg/Kg s.c. and 5 mg/Kg s.c. lessened these changes and increased the survival rate in a dose related manner. d-Bucumolol at 5 mg/Kg, on the other hand, increased survival rate primarily by suppressing ventricular fibrillation without any significant effect on other parameters. These results suggest that the membrane stabilizing action does not contribute to protective actions of bucumolol against myocardial ischemia.