Background Cardiac aging alters many of the acute responses to exercise stress, but the extent to which chronic exercise (ie, training) can alter or improve the effects of aging in humans is largely unknown.Methods and Results Cardiovascular responses to graded supine exercise stress (beginning at 200 kpm and increasing by 200 kpm every 3 minutes till exhaustion) were assessed using radionuclide ventriculography in 13 older (age, 60 to 82 years) and 11 young (age, 24 to 32 years) rigorously screened healthy men before and after 6 months of endurance training. Repeated-measures ANOVA was used to test significance. During exercise, the old group had a lesser increase in heart rate (+105% old versus +166% young), a greater increase in mean blood pressure (+35% old versus +22% young), lesser increases in ejection fraction (+3 ejection fraction units old versus +11 units young) and peak ejection rate (+62% old versus +119% young), a greater increase in end-diastolic volume index (+8% old versus -10% young), a lesser fall in end-systolic volume index (-0% old versus -32% young), and a lesser increase in cardiac index (+135% old versus +189% young) (all P<.01 young/old versus exercise stage). Stroke volume index response to exercise was not different with aging (+14% old versus +6% young, P=NS). Exercise training increased maximal oxygen intake by 21% in the older group (28.9+-4.6 to 35.1+-3.8 mL x kg sup -1 x min sup -1, P<.001) and by 17% in the young (44.5+-5.1 to 52.1+-6.3 mL x kg sup -1 x min sup -1, P<.001) and increased peak workload by 24% in the old and 28% in the young. Exercise training had no differential effects on old versus young men. Among all subjects, training significantly reduced the resting heart rate by 12% (-8 beats per minute) and increased resting end-diastolic volume index by 13% (+9 mL/M2) and resting stroke volume index by 18% (+7 mL/M[2]) (all P<.01). At peak exercise, cardiac index increased by 16% (+1.07 L x M sup -2 x min sup -1) compared with before training, which was the result of an increase in stroke volume of 18% (+7 mL/M2) (P<.001); peak heart rate was unchanged. The increase in stroke volume index at peak exercise was the result of both a 12% increase in end-diastolic volume index (+8 mL/M2) (P<.01) and an increase in ejection fraction (+3 ejection fraction units) (P<.05) at peak exercise. The increased ejection fraction at peak exercise occurred despite a 9% increase in systolic blood pressure (+18 mm Hg) (P<.01), suggesting an increase in contractility. Thus, both the young and old increased peak exercise cardiac output by use of the Frank-Starling mechanism (ie, cardiac dilatation) as well as an increase in ejection fraction.Conclusions We conclude that there is an age-associated decline in heart rate, ejection fraction, and cardiac output responses to supine exercise in healthy men. Although the stroke volume responses of the young and old are similar, the old tend to augment stroke volume during exercise more through cardiac dilatation, with an increase in end-diastolic volume (+8%) but without much change in ejection fraction (+3 ejection fraction units), whereas the young rely more on an increase in the ejection fraction (+11 ejection fraction units) with no cardiac dilatation (-10%). Despite the significant cardiovascular changes that occur in the response to a single bout of exercise with aging, adaptations to chronic exercise training were not different with aging and included improvements in maximal workload and increases in ejection fraction, stroke volume index, and cardiac index at peak exercise. (Circulation. 1994;89:1648-1655.)