AbstractIdentified crayfish visual interneurons respond to illumination with a compound EPSP of up to 40 mV. L‐gultamate, quisqualate, and kainate mimic the depolarizing action of the natural transmitter. In reduced Mg2+, N‐methyl‐D‐aspartate (NMDA) elicits a depolarization with a reversal potential (Erev) = −60 mV. Erev is independent of extracellular calcium but shifts to +4 mV if potassium conductances are blocked by intracellular CS+. The results suggest that NMDA may gate more than one class of ionic channel. The NMDA‐elicited response is enhanced and prolonged by glycine, and kynurenate competitively blocks the action of glycine. The NMDA antagonist, D‐AP7, selectively blocks the NMDA response while enhancing the EPSP. The actions of NMDA are consistent with a role in the neural mechanisms of visual adaptation. This is the first description of an NMDA receptor in an invertebrate.