The effects of SB-269970, a selective 5-HT(7) receptor antagonist, on spontaneous sleep were studied in adult rats implanted for chronic sleep recordings. SB-269970 was infused directly into the dorsal raphe nucleus (DRN) during the light phase. The 5-HT(7) receptor antagonist (0.25-1.0 mM) induced a significant reduction of REM sleep and of the number of REM periods whereas REM sleep latency was augmented. Pretreatment with the GABA(A) receptor agonist muscimol (1.0-2.0 mM), which by itself did not affect sleep variables, prevented the decrease of REM sleep induced by SB-269970 (1.0 mM). Our results indicate that the 5-HT(7) receptor is involved in the effect of DRN serotonergic (5-HT) neurons on brainstem structures that act to promote and induce REM sleep. We propose that the SB-269970-induced suppression of REM sleep is dependent upon the inhibition of GABA release in the DRN.