Free radicals are generated after head injury. These radicals rapidly react with polyunsaturated fatty acids in the cell membrane and cause membrane destruction. This process is called lipid peroxidation. Malondialdehyde (MDA) is one of the end products of lipid peroxidation, and it is a frequently used indicator of lipid peroxidation in biological tissues. Using a diffuse head injury animal model, we studied the time course of lipid peroxidation in different regions of injured rat brains. In the present study, the MDA levels were 36.7%, 41.8%, and 35.1% greater than sham at one hour after injury at the frontal, parietal, and brain stem, respectively (p < 0.0001). The MDA levels in these regions continued to increase and peaked a 4 hours after the injury. The levels slowly decreased, and by 24 hours, they were still significantly higher than the sham control's. The elevation of MDA levels was less in the striatum and the temporal regions at one hour. They were 16.9% and 13.3%, respectively (p < 0.002). The MDA levels in these two regions continued to increase even after 4 hours of injury, but the degree of elevation never exceeded 35%. The results demonstrate that there is an immediate, posttraumatic burst of MDA production, suggesting the formation of free radicals after diffuse head injury. Even though all the regions sampled show the same effect, certain regions are less affected by this diffuse head injury animal model.