The purpose of this study was to examine the behavior of the glottis during intermittent positive-pressure ventilation (nIPPV) using a two-level positive-pressure ventilator and to compare the glottic adaptation to this ventilatory mode with the one observed using volumetric ventilators, recently reported by us. Six healthy subjects were studied during both wakefulness and sleep. Their 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, and other indices. We used the controlled ventilatory mode. The expiratory pressure was kept at 4 cm H2O, and the inspiratory pressure was increased by steps from 10 to 15 to 20 cm H2O. Increases in inspiratory pressure did not always lead to increases in effective ventilation reaching the lungs. This was due to a significant narrowing of the glottis by adduction of the vocal cords in all subjects. Periodic breathing with or without apneas were common during wakefulness, but especially during sleep, representing 10.5 +/- 11% (SD) of total sleep time. We conclude that effective ventilation during nIPPV using a two-level positive-pressure ventilator in the controlled mode is less predictable and less stable than during nIPPV using volumetric ventilators.