The role of serotoninergic mechanisms in the responses of neurones in the nucleus raphe magnus to peripheral noxious stimuli was investigated in rats anaesthetized with halothane. In normal animals a large proportion of neurones responded to peripheral noxious stimuli with excitation or inhibition, the direction of the response being dependent on the spontaneous activity of the neurone. Pretreatment with p-chlorophenylalanine (pCPA; 300 mg/kg X 3 days) led to a marked reduction in the number of cells responding to peripheral stimuli. Similarly, the injection of metergoline (5 mg/kg, i.v.) in normal animals caused a reduction in the magnitude of the neuronal responses to noxious stimuli. The results are discussed with regard to the role of serotoninergic mechanisms in the control of nociceptive transmission by this nucleus.