It has recently been demonstrated [Pettus et al., J. Neurotrauma, 11 (1994) 507-522] that moderate traumatic brain injury evokes alterations in axolemmal permeability associated with rapid local compaction of axonal neurofilaments (NF). The current communication fully characterized these local NF changes, while also exploring the possibility of other related cytoskeletal abnormalities. A tracer normally excluded by the intact axolemma (horseradish peroxidase) was administered intrathecally in cats, which were then subjected to moderate/severe fluid percussion brain injury (FPI). After survival times ranging from 5 min to 6 h post-traumatic brain injury (TBI), the animals were perfused and processed for light microscopic (LM) and electron microscopic (EM) visualization of horseradish peroxidase (HRP). HRP-containing axons identified by LM, were investigated by EM in both the sagittal and coronal planes. Electron micrographs were videographically captured, digitized, and analyzed for cytoskeletal distribution. Local alterations in axolemmal permeability to HRP were detected, and consistently linked with distinct cytoskeletal changes. Within 5 min of injury, the injured HRP-containing axons displayed a significant decrease in inter-NF spacing associated with a lack of NF side arm projections. Density analysis proved a significant increase in NF packing in the HRP-containing axons, and further revealed an associated significant decrease in microtubule (MT) density. All ultrastructural changes were seen within 5 min of injury, and persisted unchanged for up to 6 h post-TBI. Collectively, these abnormalities suggest that altered axolemmal permeability triggers a rapid, yet persisting general cytoskeletal change most likely linked to local ionic disregulation. We posit that this local cytoskeletal collapse/alteration marks a site of impaired axonal transport, associated with upstream axoplasmic swelling and eventual axonal detachment.