Levels of benzodiazepine receptor ligands were measured in plasma samples from 25 patients in various stages of hepatic encephalopathy due to fulminant liver failure who were not exposed to pharmaceutical benzodiazepines immediately before or during hospitalization. Chromatographic analysis of extracted plasma samples revealed one to nine fractions containing material that competitively inhibited [3H]flumazenil binding to benzodiazepine receptors with the pharmacological properties of benzodiazepine receptor agonists. Two of these peaks were positively identified as the 1,4-benzodiazepines diazepam and N-desmethyldiazepam on the basis of chromatographic, ultraviolet and mass spectral evidence. The plasma levels of diazepam and N-desmethyldiazepam were significantly increased above control values in stage 4 hepatic encephalopathy, whereas total benzodiazepine receptor ligand concentrations were increased above control in stages 1 through 4. A significant but weak linear correlation was found between the relative increase in the levels of diazepam, N-desmethyldiazepam and total benzodiazepine receptor ligands and the severity of hepatic encephalopathy. Thus increased concentrations of benzodiazepine receptor ligands appear to contribute to the enhancement of GABAergic neurotransmission in hepatic encephalopathy, particularly in stage 4. These results constitute further support for a role for benzodiazepine receptor ligands in the pathogenesis of hepatic encephalopathy associated with acute liver failure. (Hepatology 1994;19:112-121).