Acupuncture at the Neiguan (P6) point has been shown to lessen nausea and vomiting which are related to vagal modulation. This study investigated whether acupuncture at the P6 point could improve vagal modulation by using heart rate variability analysis. We compared the heart rate variability measures of 39 subjects receiving acupuncture at the P6 point, 38 subjects receiving sham acupuncture, and 34 subjects receiving no treatment at all. The normalized high-frequency power was used as the index of vagal modulation, and the low-/high-frequency power ratio was used as the index of sympathovagal balance. The normalized high-frequency power after acupuncture increased significantly from 28.1 +/- 12.6 nu (mean +/- SD) to 30.7 +/- 14.1 nu in the P6 acupuncture group, but not in the sham acupuncture (30.6 +/- 13.7 nu versus 31.8 +/- 13.8 nu) or no-treatment group (30.1 +/- 15.0 nu versus 30.1 +/- 15.7 nu). In both the P6 and sham acupuncture groups, the mean RR interval (the intervals between consecutive R waves in the electrocardiogram) increased significantly after acupuncture. In the no-treatment group, there was no statistical difference in all heart rate variability measures in the initial and later sessions. In conclusion, acupuncture at the P6 point can increase vagal modulation of the subjects. This result may be helpful in the understanding of the mechanism underlying the effect of acupuncture or acupressure at P6 on the lessening of nausea and vomiting in clinic.