Abstract: The sulfur-containing amino acids, l-and d-cysteate, l-cysteine, l-and d-cysteine sulfinate, l-and d-cysteine-S-sulfate, l-cystine, l-and d-homocysteate, l-and d-homocysteine sulfinate, l-homocysteine, l-serine-O-sulfate, and taurine were tested in two excitatory amino acid receptor functional assays and in receptor binding assays designed to label specifically the AAl/N-methyl-d-aspartate (NMDA), AA2/quisqualate, and AA3/kainate receptor recognition sites, as well as a CaCla-dependent l-2-amino-4-phosphonobutanoate site, and a putative glutamate uptake site. Agonist efficacies were determined by chick retinal excitotoxicity and stimulated sodium efflux from rat brain slices. d-Homocysteine sulfinate, l-homocysteate, and l-serine-O-sulfate had affinities most selective for the NMDA binding site, whereas the binding affinities of d-cysteate, d-cysteine sulfinate, d-homocysteate, and l-homocysteine sulfinate were less selective. However, the correlation of agonist activity sensitive to blockade by d-2-amino-7-phosphonoheptanoate or d-2-amino-5-phosphonopentanoate in the functional assays with affinity in the NMDA binding assay (r= 0.87, p < 0.005 and r= 0.98, p < 0.005 for excitotoxicity and sodium efflux, respectively) allows characterization of these sulfur-containing amino acids as acting at NMDA subclass receptors. l-Homocysteate, which has been found in the brain, and l-serine-O-sulfate are selective agonists and could serve as endogenous neurotransmitters at the NMDA receptor.