Supine preterm infants characteristically adopt a lateral head position; however, it is not known whether this influences the distribution of nasal airflow. Ventilation was measured in 12 healthy preterm infants (postconceptional age 34 +/- 2 weeks) by employing a nasal mask pneumotachygraph that separated airflow between the left and right nasal passages. In the midline supine position, the percent of total tidal volume (%VT) through the right nasal passage ranged from 31% to 64% and varied by less than 5% between active and quiet sleep in any infant. Lateral positioning of the head caused %VT to increase on the dependent side and decrease through the upper nasal passage. When the right side was dependent, mean %VT on that side increased from 52 +/- 9% to 67 +/- 14% (P less than 0.01) and decreased to 43 +/- 10% (P less than 0.05) when the right side was up. In the midline position, the presence of a nasogastric tube caused %VT through the nasal passage with the tube to fall from 54 +/- 8% to 39 +/- 8% (P less than 0.01). The %VT fell farther, to 25 +/- 10% (P less than 0.01), when the nasal passage with the nasogastric tube was up. Despite these changes in VT distribution, total VT remained constant during these maneuvers. We speculate that when supine preterm infants adopt a lateral head position, the decrease in airflow through the upper nasal passage results from partial obstruction of the oropharyngeal or nasopharyngeal airway on that side.