We have examined the respiratory-related activation of pharyngeal constrictor (PC) muscles in decerebrate cats, normal adult humans and patients with obstructive sleep apnea. In decerebrate cats and awake normal adult humans, phasic expiratory PC activity is uniformly present under hypercapnic and hypoxic conditions. While the PC muscles are electrically silent during quiet breathing in normal adult humans in NREM sleep, an activation pattern very similar to that of other upper airway dilators, such as the genioglossus muscle, is present during spontaneous and induced apneas in patients with obstructive sleep apnea. Experiments using an isolated, sealed upper airway preparation in decerebrate cat show that selective activation of the PC muscles stiffens the pharyngeal airway. The results also show that activation of the PC muscles constricts the airway at relatively high airway volumes but dilates the airway at relatively low airway volumes. These results suggest that PC muscle activation at the end of an apneic episode, when airway volume is relatively low, may help restore airway patency in patients with obstructive sleep apnea.