The mechanisms underlying arterial blood pressure (AP) and heart rate (HR) beat-to-beat variability were investigated using spectral analysis in conscious genetically normotensive (LN) adult rats from the Lyon strain. Basal AP and HR spectra exhibited peaks in low- (LF: 0.27-0.74 Hz) and high- (HF: 0.75-3.85 Hz) frequencies. The LF oscillations of systolic AP, and even more of diastolic AP, could be attributed to the sympathetic nervous system influence as, after destruction of the peripheral sympathetic nerves, the LF peak disappeared and the LF power spectral density (PSD) was highly reduced. Ganglionic blockade with chlorisondamine combined with a restoration of the basal mean arterial pressure (MAP) level decreased LF PSD of MAP in control rats in a larger extent than sympathectomy. No relationship was found between the MAP response to chlorisondamine, taken as an index of the sympathetic vasomotor tone, and the basal LF PSD. The role of the baroreflex in the spectral characteristics of MAP and HR was also investigated. In rats with a chronic sinoaortic baroreceptor denervation (SAD), PSD of MAP was reduced in the LF band. Transfer function analysis between MAP and HR showed that, in control rats, coherence was high for frequencies surrounding the LF and HF peaks. In SAD rats, coherence in the LF band was abolished but maintained in the HF band. In conclusion, nearly 80% of the LF PSD of AP depend upon the autonomic nervous system activity and the activation of the peripheral sympathetic nerves contributes mainly to the production of LF spectral power. The baroreflex accounts for half this power and for the coherence between MAP and HR oscillations in the LF band only.