To gain insight into the morphogenetic functions of the recognition molecule tenascin in the central nervous system, we have studied its localization in the developing and lesioned adult mouse optic nerve using light and electron microscopic immunocytochemistry. Since tenascin is a secreted molecule, we have analysed the tenascin-synthesizing cells in tissue sections of retinae and optic nerves by in situ hybridization. A weak and homogeneous tenascin immunoreactivity was detectable in the developing retinal nerve fibre layer and optic nerve of 14-day-old mouse embryos, the earliest developmental age investigated. In the optic nerve of neonatal and 1-week-old animals, a high number of tenascin messenger RNA (mRNA)-containing cells were present, and antibodies to tenascin labelled the surfaces of astrocytes and unmyelinated retinal ganglion cell axons. With increasing age, expression of tenascin in the optic nerve was down-regulated at the mRNA and protein levels. At the fourth postnatal week, blood vessels in the optic nerve and collagen fibrils in the vicinity of meningeal fibroblast-like cells still showed significant immunoreactivity, but the optic nerve tissue proper no longer did so. In adult animals, tenascin was no longer detectable in association with blood vessels located in the myelinated part of the optic nerve, and meninges were only weakly immunoreactive. Also, tenascin mRNA-containing cells were no longer detectable in the myelinated part of the adult mouse optic nerve and few labelled cells were found in the meninges. In the retina, ganglion cells contained no detectable levels of tenascin mRNA at any of the developmental ages analysed. No significant up-regulation of tenascin expression was seen in the nerve tissue proper of transected proximal (i.e. retinal) and distal (i.e. cranial) optic nerve stumps of adult mice during the first 4 weeks after lesioning, the time period studied. However, collagen fibrils associated with meningeal fibroblast-like cells and located near the lesion site became strongly tenascin-immunoreactive 2 days after lesioning. Also, some blood vessels at the lesion site became immunoreactive. We conclude that tenascin in the optic nerve is synthesized by glial cells and not by retinal ganglion cells. The detectability of tenascin at embryonic ages suggests that it may mediate neurite growth in vivo. The absence of a strong, lesion-induced up-regulation of tenascin expression in the regeneration-prohibitive mouse optic nerve contrasts with the lesion-induced pronounced up-regulation in the regeneration-permissive peripheral nervous system, and may indicate a functional involvement of tenascin in regenerative processes. The high tenascin positivity of collagen fibrils at early postnatal ages and after lesioning suggests that tenascin expression may be correlated with mitotic activity of the associated meningeal fibroblast-like cells. Finally, tenascin may be involved in the process of vascularization, since the molecule is associated with blood vessels in developing and adult lesioned, but not intact adult, optic nerves.