✓ Cytochrome oxidase activities were determined by a microspectrophotometric method on microdissected samples of gray and white matter from frozen-dried sections of dog spinal cord. Experimental animals received 400 gm-cm impact at the T-3 to T-4 level. Sections for enzyme analysis and histological study were taken at the center of the trauma site, at successive 1-mm distances from the center, and at non-traumatized segments. A drop in cytochrome oxidase activities to approximately 50% of normal value was found as early as 15 minutes post-trauma, with greatest decrease at the trauma center and lesser effects at successive distances. This effect was related to known morphological defects in mitochondrial cristae and inner membranes. The enzyme change was significantly correlated with histological estimates of gray matter and neuronal damage, but was also partially independent of these processes. A small but significant increase of enzyme activity in white matter at the trauma site was considered most likely attributable to mitochondrial accumulations due to impaired axoplasmic flow.