To assess the effects of isocapnic hypoxia on the pharynx, glottis, extrathoracic trachea (ET), intrathoracic trachea (IT), and main bronchi (MB), we measured the cross-sectional areas of these airways by acoustic reflection technique in 15 healthy volunteers. Measurements were made during tidal volume breathing while subjects were normoxic [arterial O2 saturation (SaO2) > 95%] or were made hypoxic by a rebreathing procedure. Under hypoxemic conditions, airway cross-sectional areas increased significantly at ET, IT, and MB levels (P < 0.001). The magnitude of this dilation was similar for both levels of hypoxemia studied (SaO2 80-85% and 70-75%); at the milder of the two hypoxemic conditions, ET cross-sectional area increased by 12.4 +/- 4.2% (SE), IT by 10.2 +/- 5.9%, and MB by 19.1 +/- 3.2%. No significant changes were found in the pharyngeal or glottic areas. Dilation was not produced by normoxic isocapnic hyperventilation, and the use of hypoxic airway gas mixtures did not artifactually alter acoustic reflection measurements in a mechanical model. Vagal airway tone, as reflected by airway constriction during pauses in tidal breathing, was unaffected by isocapnic hypoxia. We conclude that isocapnic hypoxia produces dilation of the trachea and major bronchi, an effect unaccounted for by an alteration in the ventilatory pattern.