To evaluate effects of coenzyme Q10 added to a potassium cardioplegic solution for myocardial protection, 17 mongrel dogs underwent 60 minutes of ischemic cardiac arrest under cardiopulmonary bypass. Cardiac arrest was induced by infusing the cardioplegic solution into the aortic root every 20 minutes. Experimental animals were divided into three groups according to the cardioplegic solution used. In Group 1, we used our clinical potassium cardioplegic solution (K+, 22.31 mEq/L); in Group 2, potassium cardioplegic solution with coenzyme Q10 added (coenzyme Q10, 30 mg/500 ml of solution); and in Group 3, cardioplegic solution with coenzyme Q10 solvent. Exogenous coenzyme Q10 in the cardioplegic solution provided significantly high myocardial stores of adenosine triphosphate and creatine phosphate and a low level of lactate during induced ischemia and reperfusion. Furthermore, percent recovery of the aortic flow in Group 2 was significantly higher than that in the other two groups. Ultrastructures of the ischemic myocardium in Group 2 were better preserved than those in Group 1. Addition of coenzyme Q10 to potassium cardioplegia resulted in improved myocardial oxygen utilization and accelerated recovery of myocardial energy metabolism after reestablishment of circulation.