We have investigated the effects of iontophoretically administered gamma-D-glutamylaminomethyl sulfonate (GAMS) on excitation of dorsal horn neurons and Renshaw cells of the cat spinal cord induced by exogenous excitants and by synaptic activation following stimulation of low threshold primary afferent fibers. Comparisons were made between the synaptic depressant effects of GAMS and those of gamma-D-glutamylglycine (gamma DGG) and (+/-)-2-amino-5-phosphonovalerate (APV). At low iontophoretic ejection currents, GAMS showed clear selectivity in antagonizing responses to excitatory amino acids in the order kainate greater than quisqualate greater than L-aspartate greater than NMDA greater than L-glutamate. This selectivity was decreased at high ejection currents, when acetylcholine-induced excitation of Renshaw cells was also reduced. GAMS was equieffective with gamma DGG in depressing both APV-sensitive polysynaptic excitation and APV-resistant monosynaptic excitation of spinal neurons. Ventral root evoked excitation of Renshaw cells was not reduced by GAMS. In some cells a depression of synaptic excitation by GAMS was observed in the absence of an effect on either L-glutamate- or L-aspartate-induced excitation. This raises the possibility that some other endogenous substance may be a transmitter acting at kainate/quisqualate type receptors in the cat spinal cord. However, other factors are discussed which may explain this observation.