To investigate the molecular mechanisms involved in the outgrowth of retinal ganglion cell axons in the tectum, the expression of the extracellular matrix molecule tenascin was analysed in the tectum and retina of chickens by immunocytochemistry and in situ hybridization. Tissue was analysed between embryonic days 4 and 12, just before and during the period when retinal ganglion cell axons innervate their target region, the optic tectum. In the tectum, tenascin immunoreactivity becomes detectable at the anterior pole at embryonic day 4, 2 days before retinal ganglion cell axons arrive, and spreads caudally with increasing age. At early stages, tenascin is predominantly accumulated in the stratum opticum, the zone of ingrowing retinal ganglion cell axons, and along their prospective pathway. In the stratum opticum, the molecule is associated with radial glial fibres, glial endfeet and retinal ganglion cell axons located in the immediate neighbourhood of radial glial fibres. At all ages investigated, tenascin mRNA is mainly restricted to cells located in the periventricular region, suggesting that the molecule is synthesized by radial glial cells. In the retina, tenascin is expressed by amacrine, displaced amacrine and horizontal cells but not by retinal ganglion cells. To investigate whether the accumulation of tenascin in the developing and prospective pathway of retinal ganglion cell axons may affect their rate of growth we assayed the substrate properties of tenascin for retinal ganglion cell neurites in vitro. When retinal ganglion cell suspensions from 6-day-old chick embryos were maintained on homogeneous mouse or chick tenascin/polyornithine substrates, neurite length was significantly increased when compared to polyornithine substrates at coating concentrations of 10 or 20 micrograms/ml. Higher coating concentrations (35 or 70 micrograms/ml) resulted in neurite lengths comparable to control values. Together, these observations suggest that tenascin in the developing and prospective stratum opticum might serve as a performed pathway to support growth of retinal ganglion cell axons in the tectum.