To evaluate the difference in left ventricular function during exercise after successful aortic valve replacement, left ventricular function was investigated using radionuclide angiography in 12 patients with normal resting left ventricular systolic function. Patients were divided into two groups: Group 1 was comprised of 5 patients after aortic valve replacement for aortic stenosis and group 2 was comprised of 7 patients for aortic insufficiency. Left ventricular ejection fraction increased significantly during exercise in both groups. The increase in systolic arterial pressure to left ventricular end-systolic volume was significantly larger in group 1 than group 2, whereas the increase in left ventricular end-diastolic volume was significantly larger in group 2 than group 1. Thus, increase in left ventricular contractility played an important role in regulating increased left ventricular ejection fraction during exercise in patients with aortic prostheses for aortic stenosis, whereas increase in left ventricular end-diastolic volume played an important role in patients with aortic prostheses for aortic insufficiency.