A prospective, randomized, crossover study design was used. All infants were studied in the Infant Physiology Laboratory at Babies and Children's Hospital, New York. Two studies of approximately 6-h duration were performed starting when the infant reached full enteral intake of 180 ml/kg per day. The two studies were separated by a period of approximately 2 weeks. Each study comprised of two sequential 3-h periods of observation separated by a feeding period. Infants were randomly assigned to the supine or prone position for the first 3-h epoch; the position was then reversed in the second 3-h epoch. For the supine position, infants were placed flat on the back with the head turned to the right. For the prone position, the infants were placed in a knee-chest position with the head turned to the left. They remained in their assigned positions, supine or prone, throughout the interfeeding period, and no further manipulations were performed.