A decrease in maximal exercise heart rate (HR_max) is a key contributor to reductions in aerobic exercise capacity with aging. However, the mechanisms involved are incompletely understood. We sought to gain insight into the respective roles of intrinsic heart rate (HR_int) and chronotropic ß-adrenergic responsiveness in the reductions in HR_max with aging in healthy adults. HR_max (Balke treadmill protocol to exhaustion), HR_int (HR during acute ganglionic blockade with intravenous trimethaphan) and chronotropic ß-adrenergic responsiveness (increase in HR with incremental intravenous infusion of isoproterenol during ganglionic blockade) were determined in 15 older (65±5 yr) and 15 young (25±4 yr) healthy men. In the older men, HR_max was lower (162±9 vs. 191±11 beats/min, P<0.0001) and was associated with a lower HR_int (58±7 vs. 83±9 beats/min, P<0.0001) and chronotropic ß-adrenergic responsiveness (0.094±0.036 vs. 0.154±0.045 HR/[isoproterenol], P<0.0001). Both HR_int (r=0.87, P<0.0001) and chronotropic ß-adrenergic responsiveness (r=0.61, P<0.0001) were positively related to HR_max. Accounting for the effects of HR_int and chronotropic ß-adrenergic responsiveness reduced the age-related difference in HR_max by 83%, rendering it statistically non-significant (P=0.2). Maximal oxygen consumption was lower in the older men (34.9±8.1 vs. 48.6±6.7 ml/kg/min, P<0.0001) and was positively related to HR_max (r=0.62, P<0.0001), HR_int (r=0.51, P=0.002) and chronotropic ß-adrenergic responsiveness (r=0.47, P=0.005). Our findings indicate that, together, reductions in HR_int and chronotropic responsiveness to ß-adrenergic stimulation largely explain decreases in HR_max with aging, with the reduction in HR_int playing by far the greatest role.