Evidence suggests that endogenous benzodiazepine receptor ligands such as diazepam binding inhibitor (DBI) and its metabolite octadecaneuropeptide (ODN) may be implicated in the pathogenesis of hepatic encephalopathy. Using an immunocytochemical technique and an antibody of high specific activity to synthetic ODN, we studied the effects of portacaval anastomosis (PCA) on ODN distribution in rat brain. Four weeks after PCA, ODN immunolabeling was increased in several brain regions including cerebral cortex, hippocampus, hypothalamus and thalamus. Increased ODN immunolabeling was confined to nonneuronal elements such as astrocytes and ependymal cells. Neuropathological evaluation of brain following PCA reveals astrocytic rather than neuronal changes. These results are consistent with a role for endogenous neuropeptide ligands for astrocytic benzodiazepine receptors in the pathogenesis of hepatic encephalopathy.