Environmental insult, disease or trauma can affect the physical integrity of neuronal circuits, and the inability of many neurons to regenerate injured axons invariably leads to irreversible neural dysfunction . The conserved second messenger cyclic adenosine monophosphate (cAMP) can promote axonal re-growth [2-5]. Widely used pharmacological or genetic approaches to increase intracellular levels of cAMP are often inadequate for precise neural-circuit reconstruction because their activity cannot be easily timed to specific target cells. These shortcomings have prevented the controlled repair of pre-defined neurons at selected time points in whole specimens. Thus, technologies to guide neuronal repair in time and space would enable studies of neural-circuit recovery with unprecedented resolution. Towards this aim, we have implemented a proof-of-principle optogenetic method to promote the selective regeneration of refractory axons in a living vertebrate.