The experiment was carried out to investigate myocardial metabolism during extracorporeal circulation (ECC) and cardiac function following open heart surgery. Myocardial changes were assessed by measuring the levels of cyclic AMP, ATP, lactate and pyruvate in the right atrial myocardium and coronary blood. Myocardial cyclic AMP and ATP decreased significantly during ECC, although they remained with moderate hypothermia. The level of lactate in the myocardium rose in all cases. There was no significant correlation between ECC time and hypothermia. Serum lactate was elevated at the beginning of ECC and increased progressively during ECC, but the coronary A-V difference of lactate (⊿L) remained unchanged. The serum pyruvate level increased moderately during ECC but its A-V difference did not vary. The data suggest that lactate and pyruvate were taken up by the myocardium. There was a significant inverse correlation between cardiac index and ECC time at 2, 4 and 6 hours after operation. Stroke index showed attitude similar to cardiac index. There was a significant positive correlation between total peripheral vascular resistance and ECC time. The cardiac index and total peripheral vascular resistance with hypothermia were significantly better than those with normothermia. Patients with higher myocardial cyclic AMP and ATP at the end of ECC had better cardiac indices at 2 and 6 hours after open heart surgery.