Extrathoracic airway (ETA) stability was tested in 10 preterm infants during sleep with a drop in intraluminal pressure produced by the application of an external inspiratory flow-resistive load (IRL, 125 cmH2O.1-1.s at 1 l/min). An increase in total pulmonary resistance was sought as the measure of airway narrowing. The role of the ETA in the increased pulmonary resistance with loading was examined by testing the same infants while endotracheally intubated and after extubation. Total pulmonary resistance decreased with loading during the intubated studies (102.5 +/- 41.2 to 82.4 +/- 33.3 cmH2O.1-1.s, P less than 0.05), whereas a significant increase in pulmonary resistance was seen with loading in the extubated studies (101 +/- 58.1 to 128 +/- 68.6 cmH2O.1-1.s, P less than 0.01). Intraluminal pressure in the ETA, measured by the lowest proximal airway pressure, fell significantly with loading in both conditions, with values changing from -0.7 +/- 0.3 to -4.7 +/- 2.7 cmH2O in the intubated infants and from -0.9 +/- 0.3 to -4.6 +/- 0.9 cmH2O) in the extubated infants (P less than 0.01). The results suggest ETA narrowing with loading in extubated infants despite the absence of overt obstructive apnea. Measurements of total pulmonary resistance with IRL can be used as a simple test of ETA stability.