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1. Electromyography was used to measure the response of the diaphragm and intercostal muscles to CO2 in artificially ventilated decerebrate cats. 2. Hypocapnia produced tonic activity in either inspiratory or expiratory muscles or both, according to the preparation. 3. A graded effect of CO2 on both rhythmic and tonic activity was observed and for the latter this could be seen at as low as 10 torr PA,CO2. 4. In one human subject tonic firing of expiratory motoneurones was also induced by hypocapnia and this activity showed a graded increase with increasing (CO2. 5. A saggital incision of the medulla aimed at interrupting inspiratory bulbospinal axons abolished activity in inspiratory muscles and at eupnoeic levels of CO2 converted the activity of expiratory muscles from a periodic to a topic firing pattern. 6. Following such lesions the threshold for rhythmic excitation of expiratory muscles was elevated and this revealed that the graded effect of CO2 on tonic expiratory activity extends to as high as 60 torr. 7. The tonic activation of respiratory muscles in response to CO2 ceased after cervical cord transection or when the saggital incision in the medulla was extended caudally to the first cervical segment. 8. It is concluded that the CO2 dependent activation of spinal respiratory motoneurones is conveyed by bulbospinal axons which decussate in the vicinity of the obex and that this activation can be rhythmic or tonic. 9. It is suggested that the rhythmic excitation of expiratory muscles derives from a periodic inhibition of expiratory bulbospinal neurones which are subjected to a tonic CO2 dependent excitation which is continuously variable over the physiological range.

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