Autonomic nervous system modulation is studied non invasively by means of heart rate variability (HRV) analysis through both linear and non linear methods. Beat-to-beat intervals are computed detecting the QRS complex on the electrocardiogram and locating the R-apex using parabolic interpolation. The maximum arterial pressure within each R-to-R interval is taken as systolic arterial pressure (SAP). Sequences of 300 values are randomly selected inside each experimental condition.