Total sleep deprivation (TSD) for one or more nights produces a rapid antidepressant response in humans. Since most pharmacological treatments for depression increase brain serotonin neurotransmission, the purpose of the present study was to determine whether TSD increases the activity of serotonergic neurons in the dorsal raphe nucleus (DRN) in cats. Cats were prevented from sleeping by the experimenter, who monitored the behavioral state of each animal on a polygraph. Firing rates during quiet waking (QW) and active waking (AW) were obtained throughout a 24-h sleep deprivation period and subsequent 6-h recovery period. During the experiments, unit activity was also recorded during exposure to loud white noise, which elicited strong behavioral arousal. The inhibitory response of serotonergic DRN neurons to systemic administration of the selective 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) was determined before and after TSD to assess possible changes in 5-HT1A autoreceptor sensitivity. TSD increased mean firing rates by as much as 18% during both AW and white noise exposure. Maximal effects were observed after 15 h of TSD for AW, and after 18 h for white noise. QW firing rates also tended to be elevated throughout TSD. Firing rates for all conditions during the recovery period were not significantly different from baseline. The neuronal inhibition produced by 8-OH-DPAT was significantly diminished after TSD. Overall, these results indicate that TSD increases the firing rate of serotonergic DRN neurons during AW and arousal. This effect may be attributable to a decrease in the sensitivity of 5-HT1A autoreceptors. These findings are consistent with the hypothesis that TSD exerts its antidepressant action, at least in part, through an activation of brain serotonergic neurons.