Recent studies have suggested that activity at different 5HT receptor subtypes in the spinal cord either inhibits or facilitates nociceptive processing in the spinal cord. The present study has examined the role of 5HT3 receptors in nociceptive processing in the dorsal horn of the rat spinal cord. Using both behavioural and electrophysiological studies, 5HT3 ligands have been applied by a common route (i.e. intrathecal microinjection). In addition, only noxious heat has been used as a form of stimulation. Intrathecal injection of mCPBG (0.02-0.2 nmol) increased the responsiveness of dorsal horn neurones to noxious stimulation. In contrast, two 5HT3 antagonists (ICS 205-930 (0.015-0.15 nmol) and GR 380032F (ondansetron 34 nmol)) reduced nociceptive responses. Using doses which influenced nociceptive neuronal responses no significant change in TFL was recorded. These findings suggest that activation at 5HT3 receptor facilitates nociceptive responses of some dorsal horn neurones; these results are discussed in relation to earlier reports concerning alternative effects.