We have used the recently developed cryoculture bioassay (Carbonetto et al., J. Neurosci., 7 (1987) 610-620) to document changes during development of CNS tissular ability to support nerve fiber growth. Neuronal attachment and neurite outgrowth of purified neurons cultured on tissue sections of rat spinal cord at various stages of development were quantified. Nerve fiber growth permissivity increased during embryonic stages, reaching as postnatal days 2-4 (P2-P4) a maximum value, higher than that found on adult PNS tissue sections. This permissivity diminished rapidly thereafter, indicating that early postnatally, the nerve fiber growth supporting ability of the CNS environment shifts abruptly from an increasingly permissive mode to an increasingly non-permissive status. Furthermore, after P4, neurite outgrowth permissivity diminished in parallel on white and grey matters, whereas neuronal attachment declined much more drastically on white matter than on grey matter. This indicates that the progressive loss of spinal cord ability to support nerve fiber growth is attributable to both grey and white matters. In several instances it also appeared that neuronal adhesion was not necessarily followed by a comparable level of nerve fiber growth, suggesting that these two processes could be regulated by different factors.