The effect of alae nasi activation on nasal resistance in a group of healthy preterm infants was measured. Alae nasi activity was determined via the alae nasi electromyogram obtained from skin surface electrodes during both active and quiet sleep. Nasal resistance was calculated from airflow measured with a mask pneumotachograph and transnasal pressure drop obtained by simultaneous measurement of nasal pressure via a catheter inserted in one nostril and mask pressure. The percentage of breaths accompanied by phasic alae nasi activity was higher during active sleep than during quiet sleep (43% +/- 10% v 14% +/- 6%; P less than .005), and hypercapnic stimulation (4% CO2 inhalation) significantly increased the incidence of phasic alae nasi activity to comparable levels in both sleep states (82% +/- 8% in active sleep and 82% +/- 9% in quiet sleep). Elevation of tonic alae nasi activity also occurred more frequently during active sleep (P less than .05). The presence of either phasic or elevated tonic alae nasi activity decreased nasal resistance by 23% +/- 4% during active sleep and 21% +/- 3% during quiet sleep. This reduction in nasal resistance resulted in either a lower transnasal pressure during inspiration, a higher peak inspiratory airflow, or a combination of the two. Alae nasi activity may be an important mechanism that facilitates ventilation by reducing nasal resistance, and it may help stabilize the upper airway by preventing the development of large negative pharyngeal pressure during inspiration.