In vivo microdialysis was used to analyze the role of dorsal raphe nucleus (DRN) neurons in regulating the sleep-waking cycle. Measurements of extracellular serotonin (5-HT) were made in the DRN of freely moving adult cats before and during microdialysis perfusion of 8- hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a selective 5-HT1A receptor agonist, in artificial CSF. Behavioral state alterations were measured by simultaneous polygraphic recordings. During waking and artificial CSF perfusion of probes histologically localized to the DRN, extracellular 5-HT was 4 fmol/7.5 micro L dialysate sample. With the addition of 8-OH-DPAT (10 microM in artificial CSF) to the perfusate, 5- HT levels in the same state decreased 50%, to 2 fmol/sample (p < 0.01), presumably through 5-HT1A autoreceptor-mediated inhibition of serotonergic neural activity. Concomitantly, this 8-OH-DPAT perfusion produced a short latency, threefold increase in rapid eye movement (REM) sleep, from 10 to 30% of the total recorded time (p < 0.05), whereas waking was not significantly affected. In contrast, and suggesting DRN specificity, 8-OH-DPAT delivery through a probe in the aqueduct did not increase REM sleep but rather tended to increase waking and decrease slow wave sleep. The data on REM sleep provide the first biochemically validated and direct evidence that suppression of DRN serotonergic activity increases REM sleep, and furnish a key complement to our laboratory's in vitro data indicating that mesopontine cholinergic neurons, a target of DRN projections, are inhibited by 5-HT. The 8-OH-DPAT-induced reduction of DRN 5-HT is consistent with the hypothesis that the concomitant REM sleep disinhibition is mediated by DRN serotonergic projections to mesopontine cholinergic neurons, which other data implicate in REM sleep production.