Background: Aging results in marked abnormalities of cardiovascular regulation. Regular exercise can improve many of these age-related abnormalities. However, it remains unclear how much exercise is optimal to achieve this improvement or whether the elderly can ever improve autonomic control by exercise training to a degree similar to that observed in healthy young individuals. Methods: Ten healthy sedentary seniors (71±3 (SD) yrs) trained for 12 mo involving progressive increases in volume and intensity. Static hemodynamics were measured, and R-R interval (RRI) and beat-to-beat blood pressure (BP) variability and transfer function gain between systolic BP and RRI were calculated at baseline and every 3 mo during training. Data were compared with those obtained in 12 Masters athletes (68±3 yrs) and 11 healthy sedentary young individuals (29±6 yrs) at baseline. Additionally, the adaptation of these variables after completion of identical training loads was compared between the seniors and the young. Results: Indices of RRI variability and baroreflex gain were decreased in the sedentary seniors but preserved in the Masters athletes compared with the young at baseline. With training in the seniors, baroreflex gain and resting BP showed a peak adaptation after moderate doses of training following 3-6 mo; indices of RRI variability continued to improve with increasing doses of training, and increased to the same magnitude as the young at baseline after heavy doses of training for 12 mo; however baroreflex gain never achieved values equivalent to the young at baseline, even after a year of training. The magnitude of the adaptation of these variables to identical training loads was similar (no interaction effects of agextraining) between the seniors and the young. Conclusions: RRI variability in seniors improves with increasing "dose" of exercise over 1 yr of training. In contrast, more moderate doses of training for 3-6 mo may optimally improve baroreflex sensitivity, associated with a modest hypotensive effect; however higher doses of training do not lead to greater enhancement of these changes. Seniors retain a similar degree of "trainability" as young subjects for cardiac autonomic function to dynamic exercise.