Responses of single neurones to vibrissal stimulation and to the iontophoretically applied putative neurotransmitters, l-glutamate and l-aspartate, were recorded in the trigeminal nucleus caudalis of the rat. Vibrissal responses were unaffected by the specific N-methyl- d-aspartate receptor antagonist, d-α-aminoadipate, but were antagonized by the broad-spectrum amino acid antagonist cis-2,3-piperidine dicarboxylate. These findings suggest that the neurotransmitter of vibrissae afferent fibres may be an endogenous excitatory amino acid acting at a receptor other than the N-methyl- d-aspartate receptor.