In this paper the main fibrous proteins of the nervous system are discussed from a biochemical standpoint. The biochemical properties of the proteins making up the neurofilaments and neurotubules are briefly reviewed and attention is turned to the assembly of supramolecular structures from tubulin, the microtubular protein. Vinblastine-induced assembly is surveyed as a model for assembly and the role of guanosine 5'-triphosphate in this is noted. The in vitro assembly conditions for microtubules recently introduced by Weisenberg are recounted and the role of calcium in controlling this is noted. The role of guanosine 5'-triphosphate and the roles it may and may not play are discussed in some detail as is the role of cyclic adenosine monophosphate. The evidence presented does not support a role for phosphorylation of the protein subunit in microtubule assembly A tentative model for the in vivo control of microtubule assembly and the possible relation of cyclic adenosine monophosphate and hormones such as insulin and nerve growth factor are presented.