Peripheral nerves exit from the brain through the transition zone where oligodendroyctes and astrocytes of the central nervous system (CNS) and Schwann cells of the peripheral nervous system (PNS) are in close proximity. In this zone, the same axons are ensheathed by oligo-dendrocytes and Schwann cells. We examined, in adult rats, the expression of the low affinity neurotrophin receptor (p75) in central glia and Schwann cells in response to lesion of lumbar dorsal roots. In normal rats, scattered p75-immunoreactive glial cells were present in the CNS-PNS transition zone. A marked increase of p75 immunoreactivity occurred in Schwann cells near the transition zone from 4 days to at least 3 weeks after dorsal root transection. In contrast, the p75 immunoreactivity remained unchanged in central glia. The differential expression of p75 in the two types of glial cells was sharply demarcated at the CNS-PNS border. Our results are consistent with earlier observations that axon damage is less potent in its ability to induce central glial expression of p75, and further, suggests a possible mechanism for the failure of regenerating dorsal root axons growing into the spinal cord.