We have investigated corticospinal (CS) axon growth around cervical spinal injury in the neonatal rat and related this growth to the astroglial reaction occurring at the lesion site. Rats received a high cervical overhemisection (left dorsal funiculotomy, right spinal hemisection) and a right cortical ablation on Postnatal Days (PNDs) 0, 3, 6, 12, and 21 to 24 (weanlings). In chronic operates the remaining CS projection from the left sensorimotor cortex was then assayed using wheat germ agglutinin-horseradish peroxidase as an anterograde tracer. In other operates the formation of the astroglial scar at the spinal lesion site was studied using a monoclonal antibody to glial fibrillary acidic protein. In PNDs 0-6 operates labeled axons extend through the intact left hemicord to bypass the lesion. The labeled axons travel to the edge of the lesion, cross the midline, and pass lateral to the lesion within the dorsal and intermediate gray and dorsal lateral white matter. Axons project bilaterally to normal areas of CS termination in PND 0 operates for a distance of 2.5 to 4 spinal segments caudal to the lesion which decreases to 1.5 to 2 segments in PND 6 operates. In PND 12 and weanling operates labeled fibers do not grow around the lesion but instead are retracted rostrally. There is an astrocytic reaction to injury at all ages by 3 days postoperatively (p.o.) that becomes greater with age and p.o. survival time. A more complicated cystic scar forms in 6-day and older operates. These data show that there is an age-related change in the ability of CS axons to grow around spinal injury which ends near the time CS elongation and gliogenesis is complete in the spinal cord.