An endogenous polypeptide of rat brain has been identified that is capable of displacing 1,4-benzodiazepines and the esters of the 3-carboxylic acid derivatives of β-carbolines from their specific synaptic binding sites. This polypeptide was termed diazepam-binding inhibitor (DBI). Previous studies have shown that DBI injected intraventricularly in rodents elicits "proconflict" responses and antagonizes the "anticonflict" action of benzodiazepines. An antiserum to this peptide, directed toward an immunodeterminant near its amino terminus, makes it possible to detect, measure, and study the neuronal location of this peptide in rat brain. In the rat cerebral cortex, DBI immunoreactivity is located in neurons that are not GABAergic (GABA, γ-aminobutyric acid); in the cerebellum and hippocampus, however, it might be present also in GABAergic neurons.