Unlike mammals, lower vertebrates can regenerate their optic nerves and certain other CNS pathways throughout life. To identify the molecular bases of this phenomenon, we developed a cell culture model and found that goldfish retinal ganglion cells will regenerate their axons in response to the purine nucleoside inosine. Inosine acts through a direct intracellular mechanism and induces many of the changes in gene expression that underlie regenerative growth in vivo, e.g., upregulation of GAP-43, T alpha-1 tubulin, and the cell-adhesion molecule, L1. N-kinase, a 47-49-kDa serine-threonine kinase, may mediate the effects of inosine and serve as part of the modular signal transduction pathway that controls axon growth. In vivo, inosine stimulates extensive axon growth in the mature rat corticospinal tract. Following unilateral transection of the corticospinal tract, inosine applied to the intact sensorimotor cortex stimulated layer 5 pyramidal cells to upregulate GAP-43 expression and to sprout axon collaterals. These collaterals crossed the midline at the level of the cervical enlargement and reinnervated regions whose normal connections had been served. Further understanding of the molecular changes that lie upstream and downstream of N-kinase may lead to new insights into the control of axon growth and to novel methods to improve functional outcome in patients with CNS injury.