Reperfusion ventricular fibrillation during coronary artery bypass surgery is common and electric shocks are often needed to terminate it. Both the fibrillation and the reversing electric shocks are potentially detrimental to the myocardium. In 61 aortocoronary bypass patients with uncomplicated clinical course (no difficulties in weaning from bypass, no ECG changes and no inotropic medication), serial creatine kinase-MB values were recorded. Evaluated explanatory variables were patient age, ejection fraction, aortic occlusion time, perfusion time, number of peripheral anastomoses and of anastomoses to marginal branches, myocardial fibrillation time before aortic cross-clamping, after cross-clamping prior to cardiac arrest and after declamping, and number of defibrillations. The results indicated that reperfusion fibrillation times up to 10 minutes are not harmful, provided that left ventricular decompression is carried out. Too early and thus numerous defibrillations raise creatine kinase-MB levels and probably also damage the myocardium, and therefore should be avoided.