To study the circadian rhythm changes of the heart rate variability (HRV) during chronic sound stress, Wistar rats were implanted with telemetry transmitters and exposed to chronic ultrasound stress for 14 days. The heart rate, mean R-R intervals (mean R-R) and body temperature were monitored hourly. The spectra of five-minute heart rate variability were plotted on a log-log scale of frequency versus power spectral density, and the spectral exponent beta of the regression line of this plot was calculated. The exponent beta, heart rate (HR) and body temperature recorded hourly were plotted and fitted to sine curves to observe the circadian rhythm of these parameters. The correlation coefficient of the fitted sine curves in beta decreased from 0.644 in the control period to 0.105 in the stress period, indicating that the rhythm of beta deceased during stress. This did not occur in the other two parameters, demonstrating that the HRV exponent can assess chronic stress.