A new project using morphological techniques, immunocytochemistry and tissue culture was begun to characterize the cellular morphology, organization and function of nerves and to understand the celll interactions necessary to form and maintain nerve tissue during development and aging. The whole embryonic and adult chicken peripheral nerve was divided into cellular compartments: the perineurial nerve sheath and the endoneurial contents which were studied as cultured explants for several days. Practically all of the explant cells and filaments that intensely stained with rabbit anti-chick fibroblast vimentin and were Colcemid sensitive. Cryostat sections of adult nerve had anti-vimentin staining in the epineurial and perineurial sheath cells, Schwann cells, but not axons. These vimentin-type intermediate filaments contribute to the characteristic large numbers of filaments observed by electron microscopy in the flat, adult perineurial sheath cells. Electron microscopy of rat sciatic nerve showed large increases in collagen and ground substance in the nerve sheath and endoneurium and increases in cytoplasmic inclusions in aging Schwann cells, sheath cells and axons.