The activation of upper airway (UAW) muscles is believed to increase UAW patency to air flow. To evaluate the mechanisms by which UAW muscles act to prevent UAW collapse, pressure-flow relationships of the isolated UAW as well as the negative pressure required to cause UAW collapse (Pcrit) were assessed before and during electrical stimulation of four UAW muscle pairs in anesthetized dogs. Stimulation of each of the muscles shifted the pressure-flow curve toward lower pressures for any given flow rate, indicating UAW dilatation. UAW resistance decreased from 7.9 +/- 0.6 to 0.4 +/- 0.1, 2.7 +/- 0.6, 2.3 +/- 0.8 and to 4.8 +/- 1.5 cmH2O.L-1.sec during genioglossus, geniohyoid, sternothyroid and sternohyoid stimulation respectively (P < 0.01 in all cases). However, only genioglossus stimulation significantly increased Pcrit (from -3.4 +/- 0.6 to -12.0 +/- 1.8 cmH2O, P < 0.001). Relaxation of the genioglossus thus appears to produce the main impediment to air flow through the UAW, and contraction of this muscle improves UAW patency both by dilating the supraglottic airway and by stiffening its walls.