We have previously demonstrated that enzymatic digestion of chondroitin sulfate proteoglycan (CSPG) at the scar promotes the axonal regrowth of Clarke's nucleus (CN) neurons into an implanted peripheral nerve graft after hemisection of the spinal cord. The present study examined whether degradation of CSPG using chondroitinase ABC promoted the regeneration of CN neurons through the scar into the rostral spinal cord in neonatal and adult rats. Following hemisection of the spinal cord at T11, either vehicle or chondroitinase ABC was applied onto the lesion site. The postoperative survival periods were 2 and 4 weeks. The regenerated CN neurons were retrogradely labeled by Fluoro-Gold injected at spinal cord level C7. In the sham group, there was no regeneration of injured CN neurons in both neonatal and adult rats. Treatment with 2.5 unit/ml chondroitinase ABC in neonates resulted in 11.8 and 8.3% of the injured CN neurons regenerated into the rostral spinal cord at 2 and 4 weeks, respectively. In adults, 9.4 and 12.3%, at 2 and 4 weeks, respectively, of the injured CN neurons regenerated their axons to the rostral spinal cord. The immunoreactivity for the carbohydrate epitope of CSPG was dramatically decreased around the lesion site after treatment with chondroitinase ABC compared to sham control in both neonatal and adult animals. Our results show that axonal regeneration in the spinal cord can be promoted by degradation of CSPG with chondroitinase ABC. This result further suggests that CSPG is inhibitory to the regeneration of neurons in the spinal cord after traumatic injury.