Grafts of optic nerve were placed end-to-end with the proximal stumps of severed common peroneal nerves in inbred mice. It was found that fraying the proximal end of adult optic nerve grafts to disrupt the glia limitans increased their chances of being penetrated by regenerating peripheral nerve fibres. Suturing grafts to the proximal stump also enhanced their penetration by axons. The maximum distance to which the axons grew through the CNS tissue remained about 1.5 mm from 2-12 weeks after grafting. Schwann cells were seldom identified in the grafts. Varicose and degenerating nerve fibres were often seen within the grafts. Some varicose profiles were shown to be the terminal parts of axons within the grafts. Axons containing clusters of organelles resembling synaptic vesicles became more abundant in the longer-term grafts. Immunohistochemical studies performed on sutured grafts using a polyclonal antiserum to neurofilaments confirmed the impressions given by the electron microscopical observations. Grafts of neonatal optic nerve lacked myelin debris but were not usually penetrated by regenerating peripheral axons within a 6-week period. Sixty minutes after the intravenous injection of horseradish peroxidase, reaction product could be detected in the extracellular spaces around blood vessels in all types of living optic nerve graft. This indicates that blood-borne macromolecules could penetrate the grafts. However, the profiles of axons which were found within living optic nerve grafts had no obvious relationship to blood vessels and were usually surrounded by astrocytic processes. These results suggest that living astrocytes, rather than the absence of serum-derived trophic factors or the presence of CNS myelin, constitute the major barrier to the extension of axons and the migration of Schwann cells into CNS tissue.