During postnatal development, the visual cortex undergoes an experience-dependent refinement of its circuitry. This process includes synapse formation, as well as synapse elimination. Both mechanisms appear to be restricted to a limited 'critical period' which lasts for approximately 2 months in cats. We tested whether the termination of the critical period for cortical malleability is paralleled by changes in the growth permissiveness of the tissue. These changes may inhibit progressive reorganization of functional circuitries mediated by axon growth. Embryonic cortical neurons were cultured on unfixed cryostat sections of the visual cortex obtained from cats aged 2-50 weeks. After 2-3 days in vitro the distribution of viable cells and the percentage of neurite-bearing cells were determined and analysed with respect to the developmental age and subdivisions of the underlying tissue substrate. It was shown that cell adhesion and neurite formation are correlated with the developmental age of the substrate tissue and the time period of myelination. While embryonic neurons adhered and survived on grey and white matter tissue from 2- and 4-week-old kittens, there was a significant reduction in cell adhesion on the myelinated white matter regions of the tissue sections of older animals. Quantitative analyses showed that neurite formation by cultured neurons also became successively impaired on grey and white matter areas of tissue substrates, corresponding to the time course of the critical period for cortical malleability. On grey matter tissue this effect was most pronounced between the second and sixth postnatal weeks. The effects were not antagonized by coating the substrate sections with the growth-promoting molecule laminin. It is therefore proposed that neurite growth-inhibiting factors, most probably associated with central nervous system myelin, are gradually expressed postnatally and may contribute to the termination of the critical period in the visual cortex of cats.