The objective of this study was to evaluate 2,3,5-triphenyltetrazolium chloride (TTC) staining in the brain tissue of rats submitted to a closed head traumatic injury, in comparison to control rats not submitted to trauma. The closed head, weight drop trauma model described by Marmarou et al. (1994) was used. Animals were all sacrificed 24 h after trauma. Staining of cerebral coronal slices using TTC, coupled to image analysis software, was used to measure the level of staining. An ultrastructural study of the brain region underneath the impact zone, as well as from the correspondent region of control rats, was also done. The TTC image analysis revealed a significant decrease in the percentage of white area, in traumatized rats (mean +/- SEM 23.93% +/- 2.26, n = 4 for control, 12.13% +/- 1.72, n = 9 for traumatized rats, p <.05). The ultrastructural analysis revealed that the number of axons showing at least one mitochondrion was significantly higher in the trauma group (mean +/- SEM 49.3%, n = 4 rats, 75 photographs, 2443 axons) than in control groups (23%, n = 3 rats, 30 photographs, 6220 axons (p <.001). Another difference observed was the larger mitochondrial size in the axons of traumatized rats (mean diameter +/- SEM 0.520 +/- 0.003 microm) compared to the controlled rats (0.368 +/- 0.006 microm; p <.001). The ultrastructural observation of the traumatized brain revealed a significantly higher number of peroxisomes per photograph (mean number +/- SEM 10.58 +/- 1.18, n = 75) compared to the control group (0.19 +/- 0.08, n = 30, p <.001). The results indicate an increase of mitochondrial and peroxysomal relative mass, with a higher succinate dehydrogenase activity, 24 h after the induction of traumatic brain injury.