Injury to axons in the CNS leads to little regenerative repair and loss of function. Conversely, injury to axons in the PNS results in vigorous regrowth of severed axons, usually with restoration of function. This difference is generally attributed to a CNS environment that either cannot support or actively inhibits regeneration and/or a failure of CNS neurons to survive axotomy. One of the earliest responses of neurons to axotomy is the resealing of cut axons. A delay in resealing could affect a neuron's ability to survive axotomy and to regenerate a new axon. In the present experiments, using a dye exclusion technique, we demonstrate that following transection of a peripheral sensory nerve, axons reseal within 8--10 h, whereas following optic nerve transection complete resealing does not occur for more than 20 h. These results show that resealing of cut axons in a CNS environment is significantly delayed compared with axons in the PNS and suggest that this could contribute to the failure of CNS neurons to regenerate following injury.