Cells of neuronal morphology, expressing the 150- and 200-kDa neurofilament proteins, were generated in vitro from populations of neural cells dissociated from adult (greater than 60-day-old) mouse brain. Most of these neurons arose from dividing precursors, as demonstrated by the incorporation of [3H]thymidine during the culture period and autoradiography. Neuronal production was optimal under the conditions in which precursors were initially stimulated with basic fibroblast growth factor and then exposed to medium conditioned by an astrocytic cell line, Ast-1, in serum-free medium. Few, if any, neurons arose in control cultures or in cultures kept in serum and fibroblast growth factor. These results suggest that neuronal precursors exist in the adult mammalian brain, but they require discrete epigenetic signals for their proliferation and differentiation.