To assess the effects of age on ventricular performance, graded supine exercise tests with equilibrium radionuclide ventriculography were performed in six normal subjects of mean age 37 ± 4 years and in eight normal subjects with a mean age of 59 ± 2 years. At a standard submaximal work load, older subjects had a similar heart rate (older: 126 ± 10, younger: 128 ± 5 bpm) and systolic blood pressure responses (older: 198 ± 24, younger: 202 ± 24 mm Hg). Cardiac output counts increased appropriately in both groups during submaximal exercise. However, when expressed as percent change from resting values, the increases in cardiac output (older: 125 ± 14, younger: 75 ± 10 L/min; p < 0.05) were greater for the older subjects. The percent change in end-diastolic counts (older: 8.4 ± 5, younger: -2.8 ± 4), stroke counts (older: 26 ± 6, younger: 8.6 ± 4), and ejection fraction (older: 18 ± 3, younger: 11 ± 1%) in proceeding from rest to exercise Stage III (600 kg-m/min) was greater for the older subjects. Age-related differences in each of these measurements were significant at p < 0.05. These findings suggest that cardiac output during exercise is maintained by an increased heart rate in younger subjects, and by a combination of increased heart rate and the Frank-Starling mechanism in older individuals. Since the heart rate and mean blood pressure response to exercise were similar in both groups, the use of the Frank-Starling mechanism during exercise in older subjects suggests that age-related differences in ventricular preload are important in modulating the performance of the aging left ventricle.