AbstractThe endogenous ligand for the benzodiazepine receptor has not been convincingly demonstrated, nor is it clear what role this drug receptor actually plays in brain function. A number of putative endogenous ligands for the benzodiazepine receptor have been identified and reported in the literature. These putative ligands, at various stages of purification, have been typically identified by their ability to displace3H‐diazepam in a radioreceptor binding assay. Taken together, these studies provide useful information on the role that the benzodiazepine receptor and its ligand might play in brain function. The purpose of this paper is to review this area of research and attempt to provide a unifying hypothesis. In summary, calling this bioactive site the benzodiazepine receptor may be a misnomer, as it appears to effect both anxiogenic and anxiolytic activities involved in the maintenance of homeostatic levels of vigilance. Moreover, it can be suggested that endogenous ligands of reciprocal action could exist, mediating each function (relaxation and vigilance) and, thereby, maintain a balance of neural activit