The optic tecta of surgically produced three-eyed tadpoles were chronically exposed to the N-methyl-D-aspartate (NMDA) receptor antagonist aminophosphonovaleric acid (APV), or to NMDA itself, to assess the influence of NMDA receptor/channels on the eye-specific segregation of retinal ganglion cell (RGC) terminals that occurs whenever two retinas innervate one tectal lobe. Exposure of the tectum to the active isomer of APV produces desegregation of the RGC terminals without blocking electrical activity in the afferents or altering their terminal arbor morphology. Exposure to the inactive isomer of APV causes no perturbation of the normal stripe pattern. APV-induced desegregation is completely reversible within 2 weeks of removal of the APV. In addition, exposure of the optic tectum to NMDA results in stripes with sharper borders and fewer forks and fusions than untreated animals. These results suggest that the NMDA receptor/channel plays a role in eye-specific segregation in the three-eyed tadpole.