Phrenic nerve stimulation (PNS) applied at end-expiration allows the investigation of passive upper airway (UA) dynamic during wakefulness. Assuming that phasic UA dilating/satbilizing forces should modify the UA properties when twitches are applied during inspiration, we compared the UA dynamic responses to expiratory and inspiratory twitches (2 s and 200 ms after expiratory and inspiratory onset respectively) in 9 men (mean age 28 y). This procedure was repeated with a 2 cm mouth opening provided with a closed mouthpiece. The percentage of flow-limited (FL) twitches was significantly higher when PNS were realized during expiration than inspiration. V_Imax of FL twitches was significantly higher for inspiratory twitches (1383 ± 42 ml.s^-1 and 1185 ± 40). With mouth aperture V_Imax decreased with an increase in the corresponding pharyngeal resistance values, and the % of twitch with a FL regimen increased but only for inspiratory twitches. We conclude that 1) UA dynamics is significantly influenced by the inspiratory/expiratory timing at which PNS is applied, 2) the improvement in UA dynamic properties observed from expiratory to inspiratory PNS characterizes the overall inspiratory stabilizing effects, and 3) mouth aperture alters the stability of UA structures during inspiration.