Intracellular free Ca2+ and free radicals are increased following spinal cord injury (SCI). These can activate calpain to degrade cytoskeletal proteins leading to apoptotic and necrotic cell death. Primary injury triggers a cascade of secondary injury, which spreads to rostral and caudal areas. We tested calpain involvement in apoptosis in five 1-cm segments of rat spinal cord with injury (40 g-cm) induced at T12 by weight-drop. Animals were immediately treated with calpeptin (250 micrograms/kg) and methylprednisolone (165 mg/kg) and sacrificed at 48 hr. Untreated SCI rats manifested 68-kD neurofilament protein (NFP) degradation (indicating calpain activity), and internucleosomal DNA fragmentation (indicating apoptosis). Both calpain activity and apoptosis were highest in the lesion, and decreased with increasing distance from the lesion. Treatment decreased 68-kD NFP degradation with reduction in apoptosis in all five areas. Thus, calpeptin and methylprednisolone are found to be neuroprotective in SCI.