Recent evidence suggests that matrix metalloproteinases (MMPs) contribute to acute edema and lesion formation following ischemic and traumatic brain injuries (TBI). Experimental and clinical studies have also reported the beneficial effects of posttraumatic hypothermia on histopathological and behavioral outcome. The purpose of this study was to determine whether therapeutic hypothermia would affect the activity of MMPs after TBI. Male Sprague-Dawley rats were traumatized by moderate parasagittal fluid-percussion (F-P) brain injury. Seven groups (n=5/group) of animals were investigated: sham-operated, TBI with normothermia (37 degrees C), and TBI with hypothermia (33 degrees C). Normothermia animals were killed at 4, 24, 72 h and 5 days, and hypothermia animals at 24 or 72 h. Brain temperature was reduced to target temperature 30 mins after trauma and maintained for 4 h. Ipsilateral and contralateral cortical, hippocampal, and thalamic regions were analyzed by gelatin and in situ zymography. In traumatized normothermic animals, TBI significantly (P<0.005) increased MMP-9 levels in ipsilateral (right) cortical and hippocampal regions, compared with contralateral or sham animals, beginning at 4 h and persisting to 5 days. At 1, 3, and 5 days after TBI, significant increases in MMP-2 levels were observed. In contrast to these findings observed with normothermia, posttraumatic hypothermia significantly reduced MMP-9 levels. Hypothermic treatment, however, did not affect the delayed activation of MMP-2. Clarifying the mechanisms underlying the beneficial effects of posttraumatic hypothermia is an active area of research. Posttraumatic hypothermia may attenuate the deleterious consequences of brain trauma by reducing MMP activation acutely.