Emerging data suggest that matrix metalloproteinase-9 (MMP-9) plays a critical role in the pathophysiology of brain injury. However, the regulatory mechanisms involved in vivo remain unclear. In this study, we focus on a mitogen activated protein kinase (MAPK) pathway that may trigger MMP-9 after traumatic brain injury. We aim to show that inhibition of the extracellular signal regulated kinase (ERK) would attenuate MMP-9 levels, reduce blood-brain barrier damage, and attenuate edema after trauma induced by controlled cortical impact in mouse brain. Western blots showed that phospho-ERK was rapidly upregulated after trauma. Treatment with U0126, which inhibits MEK, the kinase upstream of ERK, effectively prevented the activation of ERK. After trauma, gelatin zymography showed an increase in MMP-9. U0126 significantly reduced trauma-induced MMP-9 levels. Correspondingly, U0126 ameliorated the degradation of the tight junction protein ZO-1, which is an MMP-9 substrate, and significantly attenuated tissue edema. At 7 days after trauma, traumatic lesion volumes were significantly reduced by U0126 compared with saline-treated controls. These data indicate that the ERK MAPK pathway triggers the upregulation in MMP-9 after trauma, and further suggest that targeting the upstream signaling mechanisms that regulate deleterious MMP-9 activity may reveal new therapeutic opportunities for traumatic brain injury.