Despite the accumulation of a large body of evidence on the function of the kainate/alpha-amino-3-5-methyl-4-iso-xazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) excitatory amino acid (EAA) receptor subtypes within the vestibular nucleus, little is known of the metabotropic receptor subtype. The aim of the present study was to examine the response of guinea-pig medial vestibular nucleus (MVN) neurons in brain stem slices to the selective metabotropic receptor agonist, 1S,3R-amino-cyclopentyl-1,3-dicarboxylate (ACPD). Sixty percent (12/20) of MVN neurones responded to 1S,3R-ACPD at a concentration of 10(-6) M, compared with 40% (8/20) and 35% (7/20) of neurones at concentrations of 10(-8) and 10(-10) M. Both increases and decreases in firing rate were observed in different neurones: in general, the magnitude of the responses was large and the duration of the responses was long. Recordings from a slice which contained only the MVN confirmed that these responses were produced by the action of the metabotropic agonist within the MVN itself. These results demonstrate that many MVN neurones have functional metabotropic receptors.