The effects of right ventricular hypertrophy and eventual right ventricular failure on autonomic nerve regulation of heart rate variability were investigated using rats with monocrotaline (MCT)-induced pulmonary hypertension. ECG signals were obtained from a radio transmitter placed into the subcutaneous pouch in the back of the male MCT-treated and control rats for 30 min every 6 h at a sample rate of 5 kHz with or without injection of atropine (2 mg/kg I.P.) or propranolol (4 mg/kg I.P.), in a room equipped with a climate controller. Heart rate (HR) and HR variability (HRV) were analyzed in each group by power spectrograms obtained by the fast-Fourier transform algorithm. The RR interval, total power (TP), low-frequency (LF) power (0.04-0.73 Hz), high-frequency (HF) power (0.73-2 Hz) and LF/HF (L/H) ratio were measured. HR was significantly increased in the MCT-treated rats (P<0.001), which also presented lower HRV than that of the control Wistar rats; TP (P<0.05) and HF (P<0.05) power, but not the L/H ratio, were significantly lower than that of the control rats. Responses of these parameters to a muscarinic antagonist (atropine: 2 mg/kg) and a beta-adrenergic antagonist (propranolol: 4 mg/kg), however, remained intact in the MCT-treated rats. Only the parasympathetic component of autonomic nervous controls of HRV was deranged in rats with MCT-induced right ventricular failure.