Diffuse axonal injury (DAI) is an important consequence of human head trauma. This experimental investigation utilized the immunocytochemical visualization of beta-amyloid precursor protein (beta-APP) to document regional patterns of axonal injury after traumatic brain injury (TBI) and to determine the importance of injury severity on the magnitude of axonal damage. Rats underwent moderate (1.84-2.11 atm) or severe (2.38-2.52 atm) parasagittal fluid-percussion (F-P) brain injury or sham procedures. At 1, 3, 7 or 30 days after TBI, rats were perfusion-fixed and sections immunostained for the visualization of beta-APP. A regionally specific axonal response to TBI was documented after moderate F-P injury. Within the dorsolateral striatum, an early increase in beta-APP-positive axonal profiles at 24 hours (h) was followed by a significant decline at subsequent survival periods. In contrast, the frequency of reactive profiles was initially low within the thalamus, but increased significantly by day 7. Within the external capsule at the injury epicenter, numbers of immunoreactive axons increased significantly at 24 h and remained elevated throughout the subsequent survival periods. At multiple periods after TBI, selective cortical and thalamic neurons displayed increased staining of the perikarya. A significant increase in the overall frequency of beta-APP profiles was documented in the severe vs moderately injured rats at 72 h after TBI. These data indicate that parasagittal F-P brain injury (a) results in widespread axonal damage, (b) that axonal damage includes both reversible and delayed patterns, and (c) that injury severity is an important factor in determining the severity of the axonal response to TBI.