To study the effects of body position (supine versus prone) on changes in cardiac inter-beat interval during quiet and active sleep, 6-h continuous electrocardiographic recordings and simultaneous minute-by-minute behavioural activity state assignments were made in 61 healthy, growing, low birthweight infants. The infants weighed 795-1600 g at birth and ranged between 30-38 wk in postconceptual age. Infants were randomly assigned to the supine or prone position for the first 3 h of each study; the position was reversed for the second 3 h. Higher heart rates and lower time and frequency domain measures of inter-beat interval variability were observed in the prone position as compared to the supine position, during both quiet and active sleep. In addition, an analysis of consecutive increases and decreases in the instantaneous heart rate revealed a lower incidence of sustained accelerations or decelerations in the prone position. Although consistent findings concerning inter-beat interval variability and sleeping position were obtained from all analytic techniques, the differences derived from analysis of consecutive inter-beat changes were the most robust. These differences in multiple measures of cardiac rate and rhythm between prone and supine positions suggest that autonomic control of the heart is altered by body position, the net effect on heart rate being increased sympathetic dominance.