The development of cold-stable microtubules (MTs) was studied in maturing rat sciatic nerves. From 4 to 10 weeks of age, there was a large increase in the proportion of tubulin associated with stable MTs. A greater fraction of axonal tubulin than nonaxonal tubulin was associated with stable MTs. The labeled tubulin remaining behind the peak of slow axonal transport was more highly associated with stable MTs than tubulin in the peak itself. Immunoassay confirmed that the sciatic nerve contains a pool of stable MTs not identical to the peak of tubulin transport. The developmental increase in MT stability is not associated with any increase in the acetylation of tubulin or with alterations in the major MT-associated proteins. One aspect of maturation of the axonal cytoskeleton may involve deposition of tubulin into stable MTs that are either stationary or moving slowly with regard to the peak of transported tubulin.