Previous studies indicate that rapid eye movement (REM) sleep deprivation facilitates pain sensitivity. Since serotoninergic raphe neurons are involved both in regulation of sleep and descending pain modulation, we studied whether spinal 5-HT receptors have a role in sleep deprivation-induced facilitation of pain-related behavior. REM sleep deprivation of 48 h was induced by the flower pot method in the rat. The pain modulatory influence of various serotoninergic compounds administered intrathecally was assessed by determining limb withdrawal response to monofilaments. REM sleep deprivation produced a marked hypersensitivity. Sleep deprivation-induced hypersensitivity and normal sensitivity in controls were reduced both by a 5-HT1A receptor antagonist (WAY-100635) and a 5-HT2C receptor antagonist (RS-102221). An antagonist of the 5-HT3 receptor (LY-278584) failed to modulate hypersensitivity in sleep-deprived or control animals. Paradoxically, sensitivity in sleep-deprived and control animals was reduced not only by a 5-HT1A receptor antagonist but also by a 5-HT1A receptor agonist (8-OHDPAT). The results indicate that serotoninergic receptors in the spinal cord have a complex role in the control of sleep-deprivation induced cutaneous hypersensitivity as well as baseline sensitivity in control conditions. While endogenous serotonin acting on 5-HT1A and 5-HT2C receptors may facilitate mechanical sensitivity in animals with a sleep deprivation-induced hypersensitivity as well as in controls, increased activation of spinal 5-HT1A receptors by an exogenous agonist leads to suppression of mechanical sensitivity in both conditions. Spinal 5-HT3 receptors do not contribute to cutaneous hypersensitivity induced by sleep deprivation.