Two groups, each consisting of six male rats, breathed 21% O2-4% CO2 or 10% O2-4% CO2, respectively, before and after carotid sinus nerve (CSN) transection. Polygraphic recording techniques served to monitor sleep and wakefulness. The effects of these gas mixtures on the sleep-waking pattern (SWP) were studied. The SWPs of the intact and CSN-transected rats breathing 21% O2-4% CO2 were no different from rats breathing air. While breathing 10% O2-4% CO2, the greatest alteration in the rat's SWP, compared with breathing 10% O2 devoid of CO2, was in the pattern of frequency of change of states, an effect unchanged by CSN transection. We conclude that CO2 added to air did not affect the rat's SWP. However, a hypoxic-CO2 gas mixture radically altered all parameters of the SWP, an effect which was centrally mediated.