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1. The effects of microiontophoretically applied glutamate, ibotenate and other related amino acids on the spike activity of feline spinal interneurones were investigated. 2. Unlike the other amino acids, which evoked excitatory responses only, ibotenate evoked slow biphasic responses (excitation-depression) from the majority of these neurones. The depressant action was associated with an increase in spike height and was reversible. 3. The ibotenate depression could be observed as a temporary reduction in background firing rate, a loss of sensitivity to other amino acids, a progressive decrease in the excitatory responses to repetitive ibotenate applications or a fading of excitation following a prolonged administration of ibotenate. 4. The onset of the excitatory effects of glutamate, quisqualate and ibotenate was relatively well fitted by diffusion equations for a continuous point source. However, radial distances for diffusion in the case of ibotenate responses were comparatively greater than those for glutamate and quisqualate. 5. The depressant action of ibotenate was not antagonized by strychnine, picrotoxin or bicuculline. 6. Some methods of quantifying the responses to excitatory amino acids are described. The excitatory potency of quisqualate and N-methyl-D-aspartate was several times greater than that of glutamate. Aspartate and ibotenate were equipotent, while alpha-aminopimelate and alpha-methyl-DL-aspartate were much weaker. 7. It is suggested that the biphasic action of ibotenate on spinal interneurones might be the result of activation of excitatory and inhibitory sites fairly remote from the cell somata where extracellular recordings were probably made.