In order to determine whether differences exist in cardiac autonomic tone between different body positions that may be used in unconscious subjects, we examined beat-to-beat heart rate variability (HRV) in volunteers lying supine, and in left lateral and right lateral positions. We studied 24 subjects, aged 20-35 years in each of the three positions on one study day, and 6 subjects in each of the three positions on each of six different study days. In both groups of subjects we observed no difference in heart rate, respiratory frequency, total power (0.02-0.45 Hz) of HRV or the proportion of power in the high (0.15-0.45 Hz), low (0.08-0.15 Hz) or very low (0.02-0.08 Hz) frequency bands among the three positions. These results suggest that there are no cardiac autonomic tone advantages to be gained by placing a person in the recovery position on one side compared with the other.