Experiments were done on monkeys in which reversible and irreversible lesions were elicited in the spinal cord by means of impact trauma applied directly to the spinal cord. Two patterns of circulatory and pathological changes were seen, related to the severity of the trauma: (1) The reversible lesion showed an initial vascular pathological disturbance which improved with time so that the cord returned to normal in appearance and histology in five days; (2) the irreversible lesion included a pathological circulatory disruption which increased with time and included venous dilation, vascular stasis, marked intramedullary hematoma formation, and edema of the cord, and, in five days, went on to liquefactive necrosis.