An impairment of cardiovascular autonomic regulation plays an important role in pathogenesis of cardiac sudden death. Altered autonomic modulation of heart rate in systemic hypertension is mainly due to elevated blood pressure. Evidence has been provided that both the baroreceptor reflex control of heart rate and the variability of the R-R interval (HRV) during sinus rhythm may be impaired in long-standing hypertension. The baroreflex is reset to operate at higher arterial pressure levels and has a reduced sensitivity to dynamic changes in arterial pressure. This results in a diminished R-R interval and reduced HRV However, an objective assessment of HRV is possible by means of spectral analysis (based on Fourier transformation) used to detect rhythmic variations .in a record of successive R-R intervals by decomposing the record into series of sinusoidal functions of different frequencies. This methodological approach results in the spectral-density function (PSD) which describes the variability of R-R intervals and its distribution with regard to frequency. In patients with longstanding hypertension reduced low-frequency spectral components of HRV are associated with an increased risk of life threatening arrhythmias and cardiac mortality. In patients with borderline hypertension, low-frequency components of HRV are higher and low-frequency components are lower than normal suggesting that autonomic modulation of heart rate is different in borderline hypertension than in long-standing disease. It is not known, however, whether the observed abnormal cardiovascular autonomic regulation is a primary feature related to systemic hypertension and precedes the onset of hypertension.