The experiments were performed on rabbits anaesthetized with urethane and chloralose. The ECG heart rate and blood pressure were recorded. The power spectrum analysis of R-R interval (RRI) was done by computer. Electrical stimulation of nucleus ambiguus (NA), depressor nerve (DN) and the peripheral end of right vagus (RV) all evoked a decrease in heart rate and blood pressure (P < 0.001), and an increase in LF, HF, LF/HF, TV and VLF (P < 0.05-0.001); The LF, HF, LF/HF, TV and VLF were significantly decreased (P < 0.05-0.01) by pretreatment with intravenous injection of atropine. The enhancement of LF during stimulating DN and NA were attenuated (P < 0.05) by prior intravenous injection of propranolol. It is suggested that vagus activity is one of the major regulator of HRV and mediates VLF. The HF is mediated only by vagi, while the LF is mediated by both vagi and sympathetic system.