The cytology and the synaptology of the nucleus isthmo-opticus have been studied in the chick embryo at three stages during (incubation days 14 and 15) and after (day 18) the neurone death period of the nucleus. The maturation process is not synchronous throughout the nucleus:the dorsal most region lags behind the rest of the nucleus. On the other hand, cell degeneration is equally distributed over the nucleus at days 14 and 15. At day 14, the nucleus shows little cellular alignment. The neuropil is stocked with immature neurites. Synaptogenesis essentially concerns axodendritic contacts, and the presynaptic partners contain only round vesicles. A few axons undergo myelination; among them, some are degenerating. At day 15, cellular alignment is much more pronounced. Synaptogenesis is very active; it concerns axosomatic as well as axodendritic contacts. A majority of the presynaptic bags contain only round vesicles, although some have a pleomorphic population of vesicles. At day 18, the histological structure of the nucleus is similar to that found in the adult. Although the neuropil presents mature profiles (development of dendritic spines, presence of synaptic glomeruli), it still contains numerous immature features (active synaptogenesis, persistent growth cones). Possible functional implications of this maturation are discussed. In particular, the fact that the nuclear neuropil is not fully developed by the end of the incubation period suggests that the accomplishment of the feedback role of the nucleus is determined by function.