The effects of diffuse brain injury on dendritic morphology in rat hippocampus and cortex were examined in this study using the recently described impact acceleration model of traumatic brain injury (Marmarou et al., 1994). Dendritic structure was visualized using immunostaining of microtubule associated protein-2 (MAP-2). Brains were studied 24, 48, and 72 h after brain injury. Results from immunohistochemistry and light microscopy indicated a time-dependent disruption of dendritic cytoarchitecture in the CA1 subregion and in the hilus of the hippocampus but not in the dentate gyrus or CA3 subregion. Similar disruption was observed in the cortical mantle overlying the hippocampus. Although disruption of dendritic structure was observed at 24 h, the most severe damage was at 48 h after injury with evidence of at least partial recovery of MAP-2 immunostaining by 72 h. In the most severe damage, dendrites appeared to be fragmented, scattered, and unaligned, consisting of irregularly spaced and darkly stained swollen segments. A mixed pattern of immunostaining was observed in somata of hilar cells, with some appearing normal while others stained only faintly, appearing to have lost their typical polygonal shape. Semiquantitative rankings confirmed these qualitative findings. Immediate post-injury behavioral evaluations of injury severity were compared to the degree of disruption of MAP-2 immunostaining. The results of this study indicate that diffuse brain injury is associated not only with axonal damage but also with injury to dendrites.