Blood pressure homeostasis and variability are the resultant of many complicated neurohumoral interactions. The autonomic nervous system plays a key role in the process. Since arterial hypertension is characterized by an increased sympathetic tone, the determination of the sympatho-vagal balance can give more insight in the autonomic nervous function in this pathology. The assessment of sympatho-vagal balance has been derived from reflex manoeuvres as the study of the baroreceptor function by phenylephrine or nitroglycerin test, by the application of negative or positive pressure around the neck or at the lower limbs or by tilting. Other reflex manoeuvres are hand-grip, cold pressure test, Valsalva manoeuvre, mental arithmetics and microneurography, providing information about the sympathetic reflex activity, and deep breathing about vagal reflex activity. These reflex tests have several limitations, because they request cooperation of the patient. Power spectral analysis of beat-to-beat blood pressure and RR-interval recordings permit to evaluate autonomic activity at baseline conditions and to separate the different components of variability which seem to reflect specific regulatory mechanisms. For the RR-interval, the high frequency component (HF) is a marker of vagal activity, while the low frequency component (LF) is a marker of sympathetic and vagal activity. The LF/HF ratio can be considered as a marker of sympatho-vagal balance. The significance of the LF and HF components of arterial blood pressure variability is less clear. Spectral analysis can be used to study the effect of antihypertensive drugs on sympatho-vagal balance.