Dorsal raphe unit activity in freely moving cats showed a slow, rhythmic discharge rate during quiet waking (X=2.82 +/- 0.17 spikes/sec), and displayed a strong positive correlation with level of behavioral arousal. Presentation of an auditory stimulus during quiet waking resulted in significant increases in unit activity of 112% and 39% during the first sec and first 10 sec after the stimulus, respectively. This effect rapidly habituated with repeated stimulus presentations. During active waking, unit activity was significantly increased by 22% as compared to quiet waking, but there was no correlation between unit activity and gross body movements. Raphe unit activity showed a significant decrease of 17% during drowsiness (first appearance of EEG synchronization) as compared to quiet waking, and then progressive decreases during the early (--34%), middle (--52%) and late (--68%) phases of slow wave sleep. During all phases of slow wave sleep, the occurrence of sleep spindles was frequently associated with a transitory decrease in unit activity. The discharge rate would typically decrease during the few seconds immediately preceding the spindle, remains at this low level during the occurrence of the spindle, and then increase immediately after the spindle. Raphe unit activity showed decreases of 81% during pre-REM (the 60 sec immediately before REM onset) and 98% during REM, as compared to quiet waking. Unit activity reappeared 3.2 sec before the end of REM, with significant increases in unit activity of 83% and 17% during the first sec and first 10 sec of unit activity, respectively, as compared to quiet waking. The results of these studies are discussed in relation to the hypothesis that serotonin may play a modulatory, rather than mediative, role in behavioral and physiological processes.