Previous studies have suggested that monosynaptic transmission between spinal primary afferent fibers and motoneurons is mediated by an excitatory amino acid, most likely glutamate or aspartate. No such comparable studies have been carried out in the trigeminal system. In an attempt to elucidate the neurotransmitter(s) mediating monosynaptic transmission between mesencephalic of V nucleus afferents (Mes V) and trigeminal jaw-closer motoneurons, the effect of iontophoretic application of excitatory amino acid antagonists on the Mes V-induced field potential, recorded in the trigeminal motor nucleus (Mot V), was examined. Application of DL-2-amino-4-phosphonobutyrate (APB) and the broad spectrum amino acid antagonists, kynurenic acid (KYN) and gamma-D-glutamylglycine (DGG), for 3-4 min reversibly reduced the amplitude of the Mes V induced field potential. The effect of APB was much greater than any of the other compounds tested. On the other hand, the specific N-methyl-D-aspartate (NMDA) receptor blocker DL-2-amino-5-phosphonovaleric acid (APV), was without effect on the field potential. Based on current-response curves for each antagonist tested, the order of potency was determined to be APB greater than KYN greater than DGG greater than APV. These antagonists were also compared with respect to their efficacy in blocking individual jaw-closer motoneuron activity induced by iontophoretic application of amino acid receptor excitants glutamate (Glut), aspartate (Asp), kainate (K), and quisqualate (Q). NMDA application was without effects on these motoneurons. The profile of activity of these antagonists on these amino acid excitants was similar to that found in other areas of the CNS by other investigators. KYN and DGG both significantly reduced responses induced by all excitants tested, whereas APB had more modest effects on K and Q excitation and was without effect on Glut and Asp excitations in most cells tested. The data suggest that an excitatory amino acid, activating non-NMDA receptors, mediates some component of synaptic transmission between Mes V afferents and jaw-closer motoneurons. The data is also consistent with the proposal made in other systems that APB blocks synaptic transmission by a mechanism other than postsynaptic receptor blockade.