Experiments were performed on rabbits anaesthetized with urethane and chloralose. The electrocardiogram (ECG), heart rate (HR), and blood pressure (BP) were recorded and the power spectrum density (PSD) of R-R interval was analysed by a computer. The low frequency component (LF) was higher during the respective stimulation of the nucleus ambiguous, depressor nerve, and the peripheral end of right vagus with a low frequency fluctuant style than with a monotonous style (P < 0.05). Intravenous infusion of phenylephrine resulted in increases of total variance of RRI (TV), very low frequency components (VLF), LF, high frequency components (HF) and LF/HF during the first 256 RRI only (P < 0.05-0.01). In order to observe the influence of different activities of sympathetic nerves on heart rate variability (HRV), DL-homocysteine (DLH) was microinjected into the vicinity of N. Paraventricularis of hypothalamus. This injection caused increases of the TV, LF, LF/HF and VLF during the rising phase of BP and HR, but no significant change occurred at the plateau period of high BP and HR. These results suggest that heart rate variability mainly reflects the fluctuation of vagal and sympathetic tone.