Heart period and arterial pressure short term variabilities contain rhythmic oscillations which might provide information on neural mechanisms regulating cardiovascular function. Continuous electrocardiographic and/or arterial pressure signals, after appropriate analogue to digital conversion, furnish the time series which constitute the basis for spectral analysis of their variabilities. Under stationary conditions, this methodology can be utilized to assess both total variability and the power and center frequency of each rhythmic component. Human physiological and animal studies support the hypothesis that the low frequency (LF) component, around 0.1 Hz, is a marker of sympathetic modulation of both R-R and arterial pressure variabilities, while the high frequency (HF) component, around 0.25 Hz, is a marker of vagal modulation of R-R variability. LF/HF ratio of R-R variability is a marker of sympatho-vagal balance. Spectral components when assessed for a 24-hour period evidence marked circadian rhythmicity with sympathetic predominance during the day and vagal predominance at night. Various pathophysiological conditions including arterial hypertension, ischemic heart disease, cardiac transplantation, congestive heart failure, Chagas' disease and diabetic neuropathy have been explored with this methodology, and a new quantitative evaluation of the alterations in sympatho-vagal balance which seem to characterize these abnormal states has been obtained. The study of cardiovascular rhythmicity, i.e. an analysis performed in the frequency domain, although based on indirect spectral markers, seems to offer a new clinical tool for the exploration of cardiovascular neural control in health and disease.