Target-derived macromolecules in the peripheral nervous system apparently play an important part in the regulation of neuronal survival and maturation during critical stages of development. Similar trophic factors could also be required in the central nervous system (CNS) for the maintenance of intracerebral connections, and neuronal cell death may result from a decrease or failure of a target-derived trophic support. Recent experiments have demonstrated the presence of factors in brain extracts or astrocytic glial cells which can support the survival of embryonic sympathetic, parasympathetic or sensory ganglionic neurones in vitro. The importance of these putative factors for neuronal survival in vivo has, however, so far not been demonstrated. In the study reported here we have used the intracerebral grafting technique to monitor the availability of survival factors in an adult CNS target area and show here that neonatal rat superior cervical cells, which have an absolute requirement for such factors for their survival, will survive in the hippocampal formation of adult rats only in the presence of a denervating lesion.