A study has been made of the effects of the selective N-methyl-D-aspartate receptor antagonist, 2-amino-5-phosphonovalerate (APV), and the broad spectrum excitatory amino acid antagonists, gamma-D-glutamylglycine (gamma-DGG), gamma-D-glutamylaminomethylsulphonate (GAMS), 4(p-chlorobenzoyl)-cis-piperazine-2, 3-dicarboxylate (pCB-PzDA) and kynurenate, have been examined on excitation evoked on neurones in the magnocellular red nucleus (m.r.n.) of the anaesthetized cat by stimulation of the interpositus nucleus (i.p.n.) and sensorimotor cortex, and by ionophoresed excitant amino acid agonists. The profile of activity of the excitatory amino acid antagonists on m.r.n. neurones was similar to that described on neurones in other areas of the central nervous system. APV selectively depressed responses to N-methyl-D-aspartate (NMDA), whereas the broader spectrum antagonists reduced responses to kainate and quisqualate as well as to NMDA. Neuronal responses to L-glutamate and L-aspartate were depressed by all the antagonists tested. I.p.n.-evoked monosynaptic responses of m.r.n. neurones were reversibly reduced by the broad spectrum antagonists, but were unaffected by APV. Cortically evoked mono- and polysynaptic excitatory responses were reversibly depressed by APV and the broad spectrum antagonist, pCB-PzDA. The action of APV corresponded with its ability to antagonize responses to NMDA. However, the cortically evoked responses appeared to be more sensitive to the actions of pCB-PzDA than to those of APV, although the former is a less effective antagonist of NMDA-induced excitation compared with APV. APV depressed excitation induced by cortical stimuli and L-glutamate and L-aspartate. However, there was no obvious correlation between the actions of the broad spectrum amino acid antagonists on synaptically evoked responses and those induced by L-glutamate or L-aspartate on the few neurones tested. These results are consistent with an amino acid being the transmitter in the interposito-rubral and cortico-rubral excitatory pathways which interacts with non-NMDA and both NMDA and non-NMDA receptors respectively. However, the identity of the transmitter acting at these receptors remains to be determined.