Our goal was to verify glottic behavior and its effects on effective minute ventilation during intermittent positive pressure ventilation applied at increasing inspiratory pressure levels through a nasal mask (nIPPV) using a two-level positive pressure ventilator (two-level IPPV) in spontaneous mode. Ten subjects were studied while awake. The spontaneous mode was used at three levels of inspiratory positive airway pressure (IPAP): 10, 15, and 20 cm H2O. The expiratory pressure was kept at 4 cm H2O. Records of spontaneous breathing without nIPPV were also performed. The glottis was continuously monitored through a fiberoptic bronchoscope. We measured, breath by breath, the widest inspiratory angle formed by the vocal cords at the anterior commissure, the corresponding tidal volume (with respiratory inductive plethysmography), the respiratory frequency and other indices. Our data during wakefulness show that inspiratory pressures of 10 and 15 cm H2O did not result in increases in effective minute ventilation with respect to spontaneous breathing. Only at 20 cm H2O of IPAP did effective minute ventilation increase. This was due essentially to a decrease in respiratory frequency with increasing pressures, offsetting increases in tidal volume at 10 and 15, but not at 20 cm H2O of inspiratory pressure. Changes in end-tidal CO2 suggest that alveolar ventilation increased due to the change in breathing pattern. Contrary to what we observed previously with either two-level IPPV used in the controlled mode, or nIPPV performed with volumetric ventilators, the glottis did not play any noticeable role in the control of effective minute ventilation.