To assess the necessary parameters for the growth of axons within the adult rat CNS, we have used intracerebral grafts of primary fibroblasts genetically engineered to express nerve growth factor (NW). Following the implantation of NGF-producing primary fibroblasts within the striatum, cholinergic axons arising from the nucleus basalis grow toward and penetrate these grafts between 1 and 8 weeks. Grafts of noninfected control cells do not elicit axon sprouting at any time. Unmyelinated axons grow into grafts of NCF-producing cells only on reactive astrocytic processes, which contribute to a surrounding glial border. From our data concerning axon growth within the adult rat CNS, we conclude that reactive astrocytes can act as conducive substrates for growing axons; and only in the presence of elevated levels of NGF will permissive substrates (eg, astrocytes) support axon growth by NGF-sensitive neurons.