Abstract: A cDNA encoding a protein with 70% amino acid identity to the previously characterized γ-aminobutyric acidA (GABAA receptor α-subunits was isolated from a rat brain cDNA library by homology screening. As observed for α1-, α2-, and α3-subunits, coexpression of this new α-subunit (α5) with a β- and γ2-subunit in cultured cells produces receptors displaying high-affinity binding sites for both muscimol, a GABA agonist, and benzodiazepines. Characteristic of GABAA/benzodiazepine type II sites, receptors containing α2-, α3- or α5-subunits have low affinities for several type I-selective compounds. However, α5-subunit-containing receptors have lower affinities for zolpidem (30-fold) and Cl 218 872 (three-fold) than measured previously using recombinantly expressed type II receptors containing either α2- or α3-subunits. Based on these findings, a reclassification of the GABAA/benzodiazepine receptors is warranted.