The aim of the present study was to examine the role of the spinal serotonergic system in the pain relieving effect of spinal cord stimulation (SCS) using a rat model of mononeuropathy. Tactile withdrawal thresholds, cold responses and heat withdrawal latencies were assessed before and after SCS. In some rats, SCS produced an attenuation of the hypersensitivity following nerve injury (SCS responding rats). When SCS was applied immediately prior to sacrifice, the 5-HT content in the dorsal quadrant of the spinal cord ipsilateral to the nerve injury was increased in SCS responding rats. But there was no change in responding rats without stimulation, or in SCS non-responding rats with or without stimulation or in controls. Immunohistochemical examination showed a high density of 5-HT stained terminals in the dorsal horn superficial laminae (I-II) in SCS responding rats following stimulation. It was also found that i.t. administration of a sub-effective dose of serotonin in SCS non-responding rats markedly enhanced the pain relieving effect of SCS on tactile and cold hypersensitivity, while there was no effect on heat hyperalgesia. This enhanced effect on tactile hypersensitivity could be partially blocked by a GABA(B) receptor antagonist (CGP 35348) but not by a muscarinic M(4) receptor antagonist (Muscarinic toxin 3) administered i.t. shortly before the 5-HT injection. In conclusion, there is evidence that the spinal 5-HT system plays an important role in the mode of action of SCS involving the activation of descending serotonergic pathways that may inhibit spinal nociceptive processing partially via a GABAergic link.