Chick neocortical cells were cultured on cryostat tissue sections of the brain. Cells preferentially attached to the gray matter of adult rat central nervous system (CNS) tissues. In contrast, they attached to any part of the brain when cultured on developing rat or mature frog brain tissues. Transection of fiber bundles at the superior cerebellar peduncle decussation of adult rat, which reportedly causes regeneration of cerebellofugal axons, made nearby white matter permissive to cell attachment. Superimposition of the gray matter of one section onto the white matter of another, converted the former into a nonpermissive substrate for cell attachment, evidence suggesting that preferential cell attachment to the gray matter of adult rat CNS is due to inhibitory factor(s) localized in the white matter. This inhibitory factor appears to be absent in frog brain and developing rat brain. These results taken together suggest possible involvement of this factor in the regulation of axonal elongation in vivo.