The sensory axons of the adult frog sciatic nerve were found to regenerate from a local test crush in vitro, providing the nerve was subjected to a conditioning lesion in vivo 10-17 days earlier. If the latter was applied distal to the test lesion, the regeneration rate was 1.1 mm/day or only somewhat slower than in vivo. The elongation rate in vitro was maintained at an unchanged rate for at least 9 days. With the use of a two-chambered culture system we could show that the growth was dependent on local protein synthesis. If the conditioning and test lesions were both applied at the same site, the outgrowth distance after 5 days was significantly longer than when the lesions were separated and also longer than the regeneration distance 5 days after a single test lesion in vivo. The results show that the growth process is influenced by local factors and can be maintained in vitro in the absence of blood-borne signals. The present system opens unique possibilities for exploring the role of potential growth factors and other conditions involved in axonal regeneration in an adult vertebrate peripheral nerve.