The effect of age on autonomic nervous system was assessed at rest and while standing using systolic blood pressure (SBP) and diastolic blood pressure (DBP), heart rate, and power spectral analysis of the time duration between 2 consecutive R waves of an electrocardiogram (RR) interval variability, as well as on plasma norepinephrine and epinephrine levels in mild to moderate hypertensive patients (DBP, 90-110 mm Hg). Patients younger than 60 years (n=57) and older than 60 years (n=32), were evaluated after a 3- to 4-week placebo period. Plasma catecholamines were measured in the supine position at rest and after 10 minutes of standing. Power spectral analysis of the RR interval variability was performed in each condition using the high-frequency (HF) band (0.15-0.4 Hz) as an index of parasympathetic activity and the low-frequency (LF) band (0.05-0.15 Hz) and LF-HF ratio to estimate sympathetic activity. The total power was calculated as the sum of LF and HF power. supine SBP was significantly higher in older patients (P<.05). SBP and DBP increased significantly only in younger patients during standing (P<.05), while the changes were smaller and nonsignificantly lower in older patients. HR was similar in both groups at rest and increased similarly during standing. Norepinephrine and epinephrine levels were similar at rest and increased similarly in both groups of patients during standing. At rest, lower LF and HF components were observed in older patients. The LF component increased less and the HF component decreased less in older patients during standing. A lower sympathetic and parasympathetic basal cardiac tone was observed at rest in older hypertensive patients. Moreover, reduced hemodynamic and sympathetic responses to standing as assessed by SBP, DBP, and the LF component of HR variability were observed in older hypertensives in the presence of a normal catecholamine response. These observations could reflect a decreased sensitivity of cardiac beta-adrenoceptors with aging.