Power spectral analysis of heart period (R-R) and arterial pressure short-term variability reveals the existence of two rhythmic oscillations: a high frequency (HF) component corresponding to respiratory activity (around 0.25 Hz) and a low frequency (LF) component corresponding to vasomotor waves (around 0.10 Hz). Further, a very low frequency (VLF) component, with a center frequency around 0 Hz, can be identified. It is generally accepted that the power of the HF component of R-R variability is a marker of vagal activity to the sinoatrial node. We have introduced the hypothesis, which is becoming progressively accepted, that the LF component of R-R variability is a marker of sympathetic modulation of the sinoatrial node, whereas the LF component of arterial pressure variability is a marker of sympathetic modulation of vasomotor tone. Moreover, we have suggested that the evaluation (in normalized units) of the LF and HF components of R-R variability (or the LF/HF ratio) can assess the state of the sympatho-vagal balance modulating the sino-atrial node in numerous physiologic and physiopathologic conditions. A large body of evidence has been gathered on the basis of multiple recordings of cardiovascular signals obtained in humans and in acute and chronic animal experiments in favor of this hypothesis.