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The effects of cytotoxic substances such as ammonia, bile acids and endotoxin, all of which increase in the circulating blood during fulminant hepatic failure (FHF), on the blood-brain barrier (BBB) permeability and development of brain edema were examined in the rats. Direct intracarotid injection of various bile acids resulted in the staining of the cerebral hemisphere with Evans blue as well as the increase of brain water contents. Elevation of ammonia was also observed in the cerebral hemisphere where the reversible opening of the BBB was induced by deoxycholate under hyperammonemic conditions. To see the synergistic significance of cytotoxic substances (ammonia, bile acid and endotoxin) under the more physiological condition as FHF, they were simultaneously injected into a peripheral vein. Brain uptake index of 14C-inulin and brain water content increased, and electron micrographs showed the swollen astrocytic foot processes surrounded brain capillary, but not opening of tight junction, the same as an animal model of fulminant hepatic failure. The results suggest that ammonia, bile acids and endotoxin might have a possible synergistic role in the pathogenesis of the brain edema, mainly cytotoxic, and vasogenic due to acceleration of vesicular transport, in FHF.