The flow capacities of arterial and saphenous vein grafts in 100 patients who had coronary artery bypass grafting were compared under exercise conditions by continuous ventricular function monitoring, which records serial beat-to-beat radionuclide data and calculates left ventricular ejection fractions every 20 seconds. Ejection fraction profiles during graded bicycle exercise were divided into four types. In type A, the ejection fraction continued to increase. In type B, the ejection fraction initially increased, but decreased during the late exercise stage. In type C, the ejection fraction did not change. In type D, the ejection fraction continued to decrease throughout exercise. A decrease in ejection fraction, observed in type B or D, is an early indicator of myocardial ischemia. Before operation, 10 patients showed type A, 30 type B, 11 type C, and 49 type D responses. After operation, 68 patients showed type A, 21 type B, and 11 type C responses. Patients were divided into three groups according the type of bypass graft. Group 1 included 21 patients with two arterial grafts and vein grafts; group 2, 61 patients with an internal thoracic artery graft and vein grafts; group 3, 18 patients with only vein grafts. All of the grafts were patent on angiography. Eight patients (38%) in group 1 and 13 (21%) in group 2 showed a postoperative type B response, but none of the patients in group 3 had a postoperative type B response (p < 0.02). Seven of 8 patients in group 1 with postoperative type B responses had only arterial grafts to the left-side coronary arteries.(ABSTRACT TRUNCATED AT 250 WORDS)