The involvement of caspases in apoptosis after spinal cord injury (SCI) was investigated in adult mouse spinal cord after contusion. Sections of spinal cord were processed for staining 7 days after SCI with the fluorescent dye Hoechst 33342, terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end-labeling (TUNEL), and immunostaining with an antibody (CM1) recognizing activated caspase-3. Caspase-3- and caspase-8-like enzyme activities were measured colorimetrically at 8 hours to 7 days after SCI using the specific substrates Asp-Glu-Val-Asp-p-nitroanilide and Ile-Glu-Thr-Asp-p-nitroanilide, respectively. Hoechst 33342 staining showed small, bright areas in fragmented nuclei. Double labeling with TUNEL plus immunostaining with cell type-specific markers identified TUNEL-positive neurons stained by anti-neuronal nuclear protein/neurons antibody, and TUNEL-positive oligodendrocytes stained by anti-cyclic nucleotide 3'-phosphohydrolase antibody. Double labeling with CM1 and cell-type specific markers similarly identified CM1-positive neurons and oligodendrocytes. Caspase-8-like enzyme activity was increased significantly on days 3 and 7 (p < 0.01), whereas caspase-3-like activity increased on day 7 (p < 0.01). Intraventricular injection of a nonspecific tetrapeptide caspase inhibitor or a specific tetrapeptide inhibitor of caspase-3 just after SCI reduced enzyme activity at 7 days. Apoptotic cells were identified with TUNEL staining in both neurons and oligodendrocytes in mice after SCI, which also showed activated caspase-3. Increased caspase-3- and caspase-8-like activity was detected in the injured spinal cord on days 3 and 7. Caspase protease activities may be involved in delayed neuronal and glial apoptosis after SCI.