Ventilatory motor output is known to influence the upper airway. Although inspiratory upper airway resistance decreases during progressive hypoxia or hypercapnia, the effects of hypoxia and hypercapnia on expiratory upper airway resistance remain unknown. In the present study, we attempted to examine whether the expiratory and the inspiratory upper airway resistances were modified in the same way by progressive hyperoxic hypercapnia or by progressive normocapnic hypoxia. Nine healthy subjects (five males, four females, 33+/-9 years) participated in the study. Inspiratory upper airway (iUAR) and expiratory upper airway resistances (eUAR) were calculated at flow 300 ml x s(-1). Both resistances were obtained during a baseline period and during progressive hyperoxic hypercapnia or progressive normocapnic hypoxia. In all subjects, iUAR and eUAR decreased significantly during hypercapnic or hypoxic challenge (P<0.05). eUAR was always lower than iUAR during hypercapnic challenge (P<0.0001) and during hypoxic challenge (P<0.0001). The authors conclude that expiratory upper airway resistance, as with inspiratory resistance, decreases during progressive hypercapnia or during progressive hypoxia. Pharyngeal dilator or constrictor muscle activities may be implicated.