The objective of this study was to determine the effects of body position (supine vs prone) on cardiorespiratory activity during quiet and active sleep in growing low birth weight (LBW) infants. The effect of postconceptional age on cardiorespiratory activity in the two positions was also evaluated. Fifty-one healthy, growing, appropriate for gestational age LBW infants (795-1600 g), ranging from 26-37 weeks in gestational age, were evaluated. All subjects were enrolled in an ongoing study of the effects of quality of dietary energy on the rate and composition of weight gain. Infants were randomly assigned to the supine or prone position for the first 3 h of the 6-h studies; the position was reversed for the second 3 h. Continuous recordings of cardiorespiratory activity were performed along with simultaneous minute by minute assignment of behavioral sleep state. Measurements of heart rate (HR), heart period variability (RR-SD), respiratory rate (f), and respiratory variability (fSD) were made each minute. Low birth weight infants had higher HR and f and lower RR-SD and fSD in the prone position compared to the supine position, during both quiet and active sleep. With increasing postconceptional age, positional differences in HR increased during quiet sleep and differences in RR-SD increased during both sleep states. These data demonstrate systematic differences in cardiorespiratory control related to body position during sleep. We speculate that such positional differences are due to variations in autonomic control, and may, in turn, contribute to variations in susceptibility to sudden infant death syndrome.