It has been suggested that serotonin neurons and their pathways may mediate sensitivity to nociceptive stimuli by activating the descending inhibitory mechanisms at the spinal cord. This antinociceptive effect may be induced by direct administration of serotonin into the cerebrospinal fluid pathways. The experiment is designed to demonstrate the changes in the tail-flick response latency after the intrathecal injection of serotonin. Serotonin, 100 or 200 microgram, administered into the lumbar intrathecal space, produced an analgesic effect for as long as 40 minutes. Behavioral and morphologic observations after serotonin injections showed no adverse reaction. It is assumed that serotonin molecules penetrate the spinal cord tissue and activate the antinociceptive serotonergic pathyways.