It is now established that the benzodiasepines exert their actions via the specific receptor site in brain that binds these drugs with high affinity. Studies in our laboratory have focused on the characterization of the receptor and the elucidation of endogenous ligands. In this regard, the purines inosine and hypoxanthine have been further characterized and new studies involving melatonin and its brain metabolite N-acetyl-5-methoxy-kynurenamine have shown that these compounds are also putative endogenous ligands for the benzodiazepine receptor. Studies involving beta-carboline-3-carboxylate ethyl ester (beta-CCE) show that this ligand reacts differently with the receptor than antagonists (benzodiazepines), in that its binding is modulated by GABA and that it is more specific for the "central type" benzodiazepine receptor. We have also shown that Beta-CCE is more specific for the central type benzodiazepine receptor since very low binding is observed in kidney membranes.