Using the airway occlusion pressure technique, control of breathing was studied in unanesthetized and anesthetized newborn rabbits breathing various gas mixtures under steady-state conditions. Independent of the gas mixture breathed, barbiturate anesthesia resulted in a reduction of ventilation. This was not due to a change in inspiratory drive since for each gas mixture breathed the pressure generated by the inspiratory muscles 0.3 s after the onset of the occluded inspiration (P0.3) remained virtually unchanged, nor could this be attributed to changes in respiratory mechanics as indicated by the fact that the relation between P0.3 and V0.3 (the volume generated 0.3 s after onset of the immediately preceding unoccluded inspiration) did not change. On the other hand, during anesthesia inspiratory time was slightly shortened as a result of a change in both central (bulbopontine) and peripheral (volume-related vagal reflex) modulation, while expiratory duration was markedly prolonged. This disproportionate increase in expiratory duration with respect to inspiratory duration was responsible for most of the depression of ventilation found in the newborn rabbits during barbiturate anesthesia.