Studies of respiratory control during sleep have revealed that multiple sites of central CO (2) chemosensitivity exist within the brainstem, and different chemosensory sites may function only during certain sleep states. In general, chemical control of respiratory function, related to both hypercapnia and hypoxia, appears to be blunted during sleep. The decline in respiratory activity during sleep is particularly marked in the muscles of the upper airway. A variety of neuromechanical factors originating in the lungs, chest wall, and upper airway also modify respiratory function during sleep. Cardiorespiratory function seems to be less stable during sleep, and arousal responses represent a final element in the control system that preserves cardiorespiratory function by terminating the sleep state and restoring more effective control mechanisms.