The role of neuronal activity in the refinement of the retinotectal projection in chick embryos was studied using tetrodotoxin (TTX) which blocks sodium channels and grayanotoxin I (GTX) which opens the channels. Optic nerve fibers were traced by the fluorescent dye DiI. The toxins were injected into the eyeball during the refinement phase of retinotectal projection (either the 13th, 14th or 15th day of incubation). A tiny crystal of DiI was placed in the temporal part of the retina. In the control embryos, fibers caudally overshooting and arborizations outside the terminal zone were found before maturation of retinotectal projection. Overshooting fibers regressed linearly, and aberrant arborizations reduced quadratically, and then the precise retinotopic map is formed by stage 44 (18th day of incubation). Both TTX and GTX interfered with the regression of overshooting fibers and arborizations outside the terminal zone. The initial action of the toxins are reverse, but their final effect may be comparable, and they may interfere with synchronous firing of the neighboring fibers. Our results emphasize a role of neuronal activity in the refinement of retinotectal projection.