Exercise-induced alterations in cardiac function during graded cycling with submaximal and maximal intensities were studied in 13 trained and 13 untrained young men. Stroke volume (SV) and stroke index (SI) at rest and during submaximal and maximal exercise, determined by impedance cardiography, were consistently greater in the trained than in the less fit group. Training-induced bradycardia was evident in the trained group at rest and during submaximal exercise. Even when SV and SI were compared at the same absolute heart rate and left ventricular ejection time, those for the trained group were markedly greater than those for the untrained. SV for the untrained group was relatively diminished above the work rate corresponding to the anaerobic threshold. The difference in SV during exercise may be attributed to inadequate filling due to the smaller stretch of myocardial fibers in diastole and/or lesser systolic emptying of the left ventricle due to the reduced myocardial contractility in systole of untrained individuals.