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In this study on 91 subjects we tested the hypothesis of an enhanced sympathetic activity in uncomplicated essential hypertension employing spectral analysis of heart rate variability. With this technique the tonic sympathetic and vagal activities and their changes are respectively assessed by the power of ~0.1 Hz (low frequency, LF) and ~0.25 Hz (respiratory linked, high frequency, HF) components of the spectrum of the beat by beat variability of RR interval. When comparing the 40 subjects with diastolic blood pressure consistently >95 mmHg (hypertensives, Ht), with 35 age-matched controls (diastolic arterial pressure <90 mmHg, Nt), we observed that LF was greater and HF smaller in Ht as compared to Nt, thus suggesting an enhanced sympathetic activity and a reduced vagal activity in Ht. Additionally, passive tilt, which in Nt enhances LF [Δ=26 ± 2 normalized units (nu)] and reduces HF (Δ=– 22 + 2, nu), produced smaller (P<0.05) changes in Ht (ΔLF=6.3 ± 2.7 and ΔHF=-7.5 ± 2.3 nu). Furthermore, the values of LF at rest and the altered effects of tilt on LF and HF were significantly correlated with the degree of the hypertensive state. Chronic ß-adrenergic blockade (atenolol 100 mg once daily for 2 weeks, n=13) reduced heart rate and blood pressure (from 162/103 to 136/88 mmHg) together with a significant diminution of LF and an increase of HF. Thus, spectral analysis of RR variability appears to be a convenient non-invasive technique to follow the progressive alterations in sympatho-vagal balance present in essential hypertension.