Infusion of bile acid such as chenodeoxycholic acid or deoxycholic acid through the carotid artery of rats produced reversible and unilateral opening of the blood-brain barrier without any tissue damage. Intravenous drip infusion of ammonium acetate during the opening resulted in severe edema of the brain. The results suggest the importance of bile acid and ammonia for the pathogenesis of brain edema frequently observed in acute hepatic failure.