An endogenous brain ligand which competes with 3H-flunitrazepine for the binding to benzodiazepine receptor has been isolated and purified to homogeneity. The purification procedures involve the liberation of the ligand into the high speed supernatant followed by ultrafiltration through a PM10 membrane (exclusion limit: 10,000-dalton) column chromatographies on Sephadex G-50, Bio-Rad P2 and finally with three times reversed phase HPLC using C18 columns. The purified endogenous ligand is heat stable, insensitive to DNAase or RNAase and contains about 5-10% amino acid residues. It has an absorption maximum at 220 nm and a minor peak at 313 nm. The exact chemical structure is unknown.