Brain edema in acute liver failure (ALF) remains lethal. The role of vasogenic mechanisms of brain edema has not been explored. We previously demonstrated that matrix metalloproteinase‐9 (MMP‐9) contributes to the pathogenesis of brain edema. Here, we show that MMP‐9 mediates disruptions in tight junction (TJ) proteins in vitro and in brains of mice with ALF. We transfected murine brain endothelial cells (ECs) with MMP‐9 complementary DNA (cDNA) using pc DNA3.1 (+)/Myc‐His A expression vector. Tissue inhibitor of matrix metalloproteinases (TIMP‐1) cDNA transfection or GM6001 was used to inhibit MMP‐9. ALF was induced in mice with azoxymethane. Endogenous overexpression of MMP‐9 in brain ECs resulted in significant degradation of the TJ proteins occludin and claudin‐5. The alterations in TJ proteins correlated with increased permeability to fluorescein isothiocyanate–dextran molecules. The degradation of TJ proteins and the increased permeability were reversed by TIMP‐1 and GM6001. Similar results were found when MMP‐9 was exogenously added to brain ECs. We also found that TJ protein degradation was reversed with GM6001 in the brains of mice with ALF. Conclusion: TJ proteins are significantly perturbed in brains of mice with ALF. These data corroborate the important role of MMP‐9 in the vasogenic mechanism of brain edema in ALF. (HEPATOLOGY 2009.)