Previous work suggested that brain NGF acts locally on cells adjacent to sites of synthesis, in addition to any putative actions on distant, projecting perikarya. To define the basis of local action, we used a sensitive nuclease protection assay to identify cells expressing the NGF gene in vivo and in vitro. In addition to neurons, glia from a variety of developing brain areas synthesized NGF mRNA, suggesting that CNS glia exhibit a generalized capacity to express the gene. Expression was associated with active glial growth. Stimulation of growth with serum increased NGF message 2-fold in culture. Moreover, rapidly growing, low-density glial cultures exhibited 8-fold higher levels of NGF mRNA than quiescent, confluent cultures. The optic nerve, which contains all 3 major types of glia, expressed the message in vivo during neonatal development. In contrast, expression was barely detectable in the adult optic nerve. Transection, which induces glial proliferation, elicited de novo appearance of NGF mRNA in the adult nerve. Our observations suggest that active glial growth is associated with expression of the NGF gene and raise the possibility that actively growing glia in the developing or injured brain regulate neuronal growth through the elaboration of NGF.