1. Extracellular recordings of the responses of single caudal trigeminal nucleus neurones to non-noxious and noxious facial stimuli and to ionophoretically applied L-glutamate, L-aspartate and acetylcholine were made in urethane anaesthetized rats. 2. Neurones excited by non-noxious mechanical stimuli were located primarily in the magnocellular part of nucleus caudalis, whereas neurones excited by both noxious and non-noxious stimuli were located either ventromedially to the magnocellular part of nucleus caudalis or superficially to the substantia gelatinosa. 3. Both L-aspartate and L-glutamate were found to excite all neurones tested in nucleus caudalis. In contrast, however, acetylcholine was found to excite only 31% of the neurones tested. 4. Responses of nucleus caudalis neurones to non-noxious sensory stimulation were not antagonized by the excitatory amino acid antagonist D-alpha-aminoadipate, but were antagonized by cis-2, 3-piperidine dicarboxylate and gamma-D-glutamylglycine, two excitatory amino acid antagonists with a broader spectrum of action. 5. It is concluded that the chemical synaptic transmitter of non-nociceptive mechanoreceptive primary afferent fibres to nucleus caudalis may be a ligand for an excitatory amino acid receptor other than a D-alpha-aminoadipate-sensitive receptor. The synaptic receptor may thus be of the kainate or quisqualate type, and the transmitter possibly L-glutamate, L-aspartate or an as yet unidentified substance.