DL-alpha-Aminoadipate (DLalphaAA) and L-glutamic acid diethylester (GDEE) were compared as antagonists of the excitatory effects of a number of amino acids on interneurones, and also of acetylcholine on Renshaw cells of the cat spinal cord. The excitants could be ranked in order of their sensitivity of the two antagonists. Ibotenate and the optical isomers of N-methylaspartate were the most readily antagonised by DLalphaAA and the least by GDEE; L-glutamate and quisqualate were most affected by GDEE and least by DLalphaAA; the actions of L-cysteate, L-aspartate and D- and L-homocysteate were reduced to some extent by both; and kainate was relatively insensitive to both antagonists. Acetylcholine excitations of Renshaw cells were often reduced by GDEE and occasionally by DLalphaAA. The results indicate the existence on spinal neurones of at least two populations of amino acid receptors which have differing sensitivities to the antagonists, and which do not precisely conform to the "glutamate-perferring" and "aspartate-preferring" categories which have been proposed on other grounds.