Microiontophoretic applications of ibotenate, an amino acid analogue of glutamate, evoked biphasic responses from single dorsal horn interneurones of the cat spinal cord, i.e., an initial increase in the firing rate followed by a sustained depression of spontaneous firing. A reduced cell sensitivity to excitatory amino acids or peripheral field stimulation was also found during the ibotenate-induced depression. These effects were not produced by glutamate, quisqualate, or kainate, although occasional transient reductions of spontaneous cell firing were observed after application of glutamate. It is suggested that ibotenate might act on inhibitory as well as excitatory receptors of cat spinal interneurons.