In the adult mammal, nerve fibres do not regrow into the spinal cord after a dorsal root lesion. The elongation of dorsal root nerve fibres into the spinal cord of neonatal rats was examined: L4 and L5 dorsal roots were crushed in rat pups. After 3-6 months, the dorsal root-spinal cord junction was investigated morphologically in several long series of ultrathin cross-sections. In rats which had been operated on at birth (0-2 days old), axons from the lesional roots could be followed into the CNS tissue of the spinal cord. In contrast to normal development, the usual short segment of CNS glia did not grow into the neonatally lesioned roots. Instead, the CNS-PNS border was located within the spinal cord. The nerve fibres, which were of normal diameter, had regrown across the PNS-CNS border and elongated further into the CNS environment of the spinal cord. In rats operated on at the end of the first postnatal week or later, the largest dorsal root nerve fibres were only half the size of those in unoperated animals and reinnervation of the spinal cord had not occurred. An astrocyte-dominated CNS segment had developed in these roots. The impact of an early neuronal lesion on the development of certain glia cells and their importance in the outcome of spinal cord reinnervation are discussed.