OBJECTIVE: We investigated the effects of conscious sedation by 30% nitrous oxide inhalation on the cardiac autonomic nervous system in the absence or presence of music listening. STUDY DESIGN: The power spectral analysis of heart rate variability (HRV) and cardiovascular parameters were measured in 26 healthy volunteers, who were divided into the control and N(2)O groups. RESULTS: Nitrous oxide inhalation attenuated the increase in relative low frequency (LF) power in the control group, but relative high frequency (HF) power remained constant compared to a decreased value in the control group. Nitrous oxide inhibited the increase in the LF/HF ratio. These autonomic balance shifts were observed without hemodynamic changes. Additional music during sedation did not alter HRV variables. CONCLUSIONS: Inhalational sedation with nitrous oxide shifted cardiac sympathetic-parasympathetic balance toward a parasympathetic dominance via the suppression of sympathetic activity. Music listening during sedation did not provide further relaxing effects.