In order to determine the specific site of inspiratory narrowing within the upper airway during sleep, we measured supralaryngeal, oropharyngeal, and nasopharyngeal pressures and inspiratory flow in 11 healthy nonsnoring male subjects awake and in NREM sleep. Resistance was calculated at 0.01 L/s, a point along the linear portion of the pressure-flow relationship, and at peak inspiratory pressure, a point within the curvilinear section of the pressure-flow relationship. During sleep, nasal resistance increased minimally. At peak inspiratory pressure, both transpalatal and hypopharyngeal resistances increased more than 700% in NREM sleep. At 0.01 L/s inspiratory flow, transpalatal and hypopharyngeal resistances increased 200 and 400%, respectively. Six subjects had a greater increase in transpalatal than hypopharyngeal resistance, and five subjects had a greater increase in hypopharyngeal than transpalatal resistance. Three subjects in each of these two subgroups had an increase in resistance exclusively across the palate or the hypopharynx. The site of increased resistance during sleep was not predictable from awake resistance measurements. From these data, we conclude that the site of inspiratory narrowing within the upper airway during sleep occurs primarily at either the level of the palate or hypopharynx and is variable among subjects. The pattern of palatal or hypopharyngeal narrowing is the same as that observed in obstructive sleep apnea patients, but quantitatively different.