The present study compared autonomic nervous function in Kob [Spontaneously Diabetic, Bio-Breeding (BB)] rats with control Wistar rats to determine the development of cardiac neuropathy in diabetic rats. Telemetric ECG signals were obtained from an ECG radio-transmitter placed in a dorsal subcutaneous pouch of male Kob and Wistar rats for 30 min every 6 h at a sample rate of 5 kHz. Heart rate (HR) and HR variability (HRV) were analyzed in each group by power spectrograms obtained by a fast Fourier transform algorithm. RR interval, total power (TP), low frequency (LF) power (0.04-0.67 Hz), high frequency (HF) power (0.79-1.48 Hz) and LF/HF ratio were also measured. The Kob rats had lower HRV than the control Wistar rats; HR, TP, and HF power, but not the LF/HF ratio, in the Kob rats were significantly lower than those of the control rats (p<0.001). However, in the Kob rats the response of these parameters to a muscarinic antagonist (atropine: 2 mg/kg) was left intact, but their response to a β-adrenergic antagonist (propranolol: 4 mg/kg) was impeded. Autonomic nervous control of HR in spontaneously diabetic rats was inhomogeneously deranged in terms of the balance in sympathetic and parasympathetic tone, not only in the baseline condition, but also in the regulatory systems, including postsynaptic receptor function. (Circ J 2002; 66: 283 - 288)