1. Ventilation has been studied during hypocapnia produced by passive mechanical ventilation in ten normal human subjects. 2. During wakefulness, disconnection of the ventilator led to inconsistent apnoea of only brief duration. During sleep, at a similar degree of hypocapnia, disconnection of the ventilator led more consistently to apnoea which was also of much longer duration; the deeper the sleep stage, the longer the apnoea. 3. The resumption of breathing during sleep could precede or follow arousal or be unaccompanied by arousal; in the absence of prior arousal, the evidence suggests that a starting end-tidal CO2 pressure (PET, CO2) less than 41 mmHg could result in an apnoea during sleep stages I and II. 4. Subjects did not report any common sensation which led them to breathe following an apnoea whilst awake. 5. Prior hyperoxia in one subject prolonged the apnoea duration in both slow-wave sleep and rapid eye movement sleep. 6. The results are interpreted as showing that even during light sleep, the maintenance of the respiratory rhythm is critically dependent on the arterial CO2 and O2 tensions. During wakefulness, other behavioural drives, which may not reach consciousness, supervene.