The internal mammary artery has been advocated for use in coronary artery bypass grafting because of its excellent long-term patency. We compared the flow capacities of internal mammary artery and saphenous vein grafts under exercise conditions by means of radionuclide angiocardiography. Fifty-two patients were divided according to the type of bypass graft done to the left anterior descending artery: group 1 comprised 27 patients with the internal mammary artery graft, and group 2 included 25 patients with the saphenous vein graft. Saphenous vein grafts were placed into the right and circumflex systems. Before the operation, global and regional ejection fractions decreased similarly in both groups with exercise. After the operation, the global ejection fraction measured in groups 1 and 2 increased significantly from 54% +/- 2% to 57% +/- 2% and from 54% +/- 1% to 60% +/- 2%, respectively, the anteroseptal ejection fraction from 29% +/- 1% to 32% +/- 2% and from 29% +/- 1% to 35% +/- 1%, respectively, and the apical ejection fraction from 75% +/- 3% to 82% +/- 2% and from 77% +/- 2% to 86% +/- 2%, respectively. There were no differences in exercise-induced increases in the global and regional ejection fractions between groups 1 and 2. Six patients in group 1 had exercise-induced wall motion abnormalities at the anteroseptal and/or apical segments. In contrast, patients in group 2 had no exercise-induced wall motion abnormalities at these segments (p less than 0.05, group 1 versus group 2). Results of this study show that internal mammary artery grafts respond to the increased blood flow demand of exercise in essentially the same way as saphenous vein grafts. However, there seems to be a slightly greater potential for inadequate flow in patients with the internal mammary artery graft, as evidenced by the small group of patients with exercise-induced wall motion abnormalities.