1. A range of compounds has been tested for excitatory amino acid agonist or antagonist activity and for effects on synaptic activity on isolated hemisected spinal cords of frogs. 2. L-Monoamino dicarboxylic acids of chain length up to 8 carbon atoms (L-alpha-aminosuberate) were all agonists. 3. Within a series of D-monoamino dicarboxylic acids, and with diamino dicarboxylic acids (mainly unresolved mixtures of diasteroisomers), there was a progression from agonist activity, for compounds of chain length equal to or shorter than glutamate, to antagonist activity, for compounds of longer chain length equal to or shorter than glutamate, to antagonist activity, for compounds of longer chain length, D-alpha-Aminosuberate (D alpha SD) was the most potent antagonist. 4. The antagonist actions of these substances showed a Mg2+--like selectivity with respect to depolarizations produced by different excitants. N-methyl-D-aspartate (NMDA) was the most susceptible agonist and quisqualate and kainate the least susceptible. Responses to other excitatory amino acids, including L-glutamate and L-aspartate, showed intermediate sensitivity to the antagonists. 5. A parallelism was observed between the relative potencies of mono- and diamino dicarboxylic acids as NMDA antagonists and their relative potencies as depressants of synaptic responses. 6. The results support the concept of different types of excitatory amino acid receptors, with NMDA and its antagonists acting predominantly on one type. These NMDA receptors are probably transmitter receptors activated by an excitatory amino acid transmitter.