BACKGROUND: Beat-to-beat heart rate variability (HRV) is caused by the fluctuating balance of sympathetic and parasympathetic tone. Analysis of heart rate variations may provide important clinical information and the influence of anesthesia on the autonomic nervous system. METHODS: We studied 18 ASA I female patients undergoing propofol-O2-N2O anesthesia for transabdominal hysterectomy and analyzed the HRV using power spectral analysis (PSA) at preinduction period, 2~5 minutes after induction, and 30~33 minutes after skin incision. RESULTS: The powers in all frequency bands decreased, but the decrease in low+middle frequency power was less than that in high frequency power during 2~5 minutes after induction. At 30~33 minutes after skin incision, all frequency bands and ratio of low middle/high frequency power spectra recovered and were not significantly different from those of before induction. CONCLUSIONS: Propofol-N2O-O2 anesthesia depress autonomic nervous system with relative preservation of sympathetic components. However autonomic nervous system and sympathetic/parasympathetic balance during operation recovered to the pre-anesthetic pattern, probably due to surgical stimuli and stress.