A comparison of the potencies of L-glutamic and L-aspartic acids as excitants of Renshaw cells and of interneurones monosynaptically innervated from the dorsal roots in the spinal cord of the rat has been made. Both groups of cells were more sensitive to glutamate than to aspartate, and three different non-parametric statistical tests showed that the two populations were indistinguishable. The results differ qualitatively from those obtained in the cat, and for the Renshaw cells from another study in the rat.