Tetrodotoxin was injected into one eye of chick embryos so as to block activity in the target territory of the isthmo-optic nucleus (ION) during its period of neuronal death. This markedly reduced the neuronal death and thereby prolonged the survival of some 'aberrantly' projecting neurons which would normally all die. In addition, the cytoarchitecture of the ION developed abnormally. Since these two effects differ markedly in their dose-dependence and in other ways, they cannot both be explained by changes in the strength of a single retrograde signal.