Contusion injuries of the rat thoracic spinal cord were made using a standardized device developed for the Multicenter Animal Spinal Cord Injury Study (MASCIS). Lesions of different severity were studied for signs of endogenous repair at times up to 6 weeks following injury. Contusion injuries produced a typical picture of secondary damage resulting in the destruction of the cord center and the chronic sparing of a peripheral rim of fibers which varied in amount depending upon the injury magnitude. It was noted that the cavities often developed a dense cellular matrix that became partially filled with nerve fibers and associated Schwann cells. The amount of fiber and Schwann cell ingrowth was inversely related to the severity of injury and amount of peripheral fiber sparing. The source of the ingrowing fibers was not determined, but many of them clearly originated in the dorsal roots. In addition to signs of regeneration, we noted evidence for the proliferation of cells located in the ependymal zone surrounding the central canal at early times following contusion injuries. These cells may contribute to the development of cellular trabeculae that provide a scaffolding within the lesion cavity that provides the substrates for cellular infiltration and regeneration of axons. Together, these observations suggest that the endogenous reparative response to spinal contusion injury is substantial. Understanding the regulation and restrictions on the repair processes might lead to better ways in which to encourage spontaneous recovery after CNS injury.