We have studied the effects of transitory changes in cerebral state during drowsiness on breath duration and lung volume in eight healthy subjects in the absence of changes in airway resistance and fluctuations of ventilation and CO₂ tension characteristic of the onset of non-rem rapid eye movement sleep.chemical feedback. A volume cycled ventilator in the assist control mode was used to maintain CO₂ tension (PETCO₂) close to that when awake. Changes in cerebral state were determined by the EEG on a breath by breath basis and classified as alpha or theta breaths. Breath duration (TTOT) and the pause in gas flow between the end of expiratory air flow and the next breath (TP) were computed for two alpha breaths, which preceded a theta breath and for the theta breath itself. The group mean (SD) results for this alpha to theta transition was associated with a prolongation in TTOT from 5.2 (1.3) to 13.0 (2.1) s and TP from 0.7 (0.4) to 7.5 (2.2) s. Because the changes in arterial CO₂ tension (PaCO₂) are unknown during the theta breaths, we made in two subjects a continuous record of PaCO₂ in the radial artery. PaCO₂ remained constant from the alpha breaths through to the expiratory period of the theta breath by which time the duration of breath was already prolonged, representing an immediate and altered ventilatory response to the prevailing PaCO₂. In the eight subjects the PETCO₂ awake was 39.6 (2.3) mmHg and on assisted ventilation 38.0 (1.4). We conclude that the ventilatory instability recorded in the present experiments is due to the apneic threshold for CO₂ being at or just below that when awake.