Endothelial alterations occur as early as 1 1/2 minutes following impact injury to the primate spinal cord. Separation of the endothelial junctions and exposure of microvascular basal lamina result in platelet adhesion and aggregations that cover defects in the vessel wall and may progress to complete vascular occlusion. This occurs during the first six hours following injury. Platelets also adhere to the surface of damaged endothelium. Hemostasis resulting from platelet thrombus formation is responsible in part for decreased blood flow in the central gray matter following spinal cord trauma.