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The factors required to stimulate short-term Schwann cell DNA synthesis in serum-free medium (containing insulin) are thoroughly analysed and compared with the growth factor requirements of long-term ''autocrine" Schwann cells. Evidence is presented to demonstrate the importance of cAMP in Schwann cell DNA synthesis. Measurements of intracellular cAMP levels by radioimmunoassay in both Schwann cell types is related to the differing mitogen requirements of these cells. Further tissue culture studies show that insulin-like growth factor-I (IGF-I) plays a role in Schwann cell DNA synthesis, and that cultured Schwann cells are IGF immunoreactive. Binding studies and Scatchard analysis using 125I-IGF-I provide evidence for a type 1 IGF receptor on cultured Schwann cells. An immunohistochemical analysis on the distribution of IGF in vivo in the rat sciatic nerve is reported. Dried cell preparations and and 2hr cultures are used to document the presence of IGF in the Schwann cells from embryonic to adult rats. Teased nerve preparations from postnatal rats are used to study the distribution of IGF in neurones. Results obtained suggest that IGF-I may be acting as an autocrine/paracrine Schwann cell mitogen in vivo. An immunohistochemical investigation reveals that the growth associated protein GAP-43, hitherto considered to be associated with neurones and certain CNS glia, is present in non-myelin-forming Schwann cells. Nerve sections and dried cell preparations are used to document the developmental distribution of GAP-43 in Schwann cells in vivo. Further analyses on the developmental regulation of GAP-43 expression in Schwann cells has been carried out in tissue culture. Gel electrophoresis and western blotting techniques are used to confirm the identity of Schwann cell GAP-43. Evidence that GAP-43 is widely distributed in the neurones of the adult peripheral nervous system (PNS) is provided. Immunohistochemical studies and western blotting demonstrate that GAP-43 is present in high amounts in all three sub divisions of the autonomic nervous system (ANS) of the adult rat. Taken together with known distribution of GAP-43 in areas of the CNS associated with plasticity, these findings suggest a role for GAP-43 in the plasticity of the ANS.