g-Aminobutyric acid (GABA) receptors type A (GABAAR) form a heterogeneous group of ligand-gated ion channels responsive to GABA. From the 19 subunits, grouped into the classes a1-6, b1-3, g1-3, d, e, r1-3 and p, a huge number of receptor isoforms can be build, given the fact that all GABAAR are heteropentamers, with the presumed exception for the r1-3 subunits, which might form homopentamers. In the CNS the most abundant receptor isoforms are of the form a1b1-3g2, whereas in the cerebellum the subunit combination a6b2d seems to play an important role. The subunits are differently expressed in time and neurons, i.e., some appear early during development in many brain areas (a5) and disappear later on, while other subunits are present in very restricted neuronal populations only in later postnatal stages (a6). Besides this, receptor isoforms may differ substantially in the response characteristics to GABA and GABA mimetics as measured in vitro by patch-clamp analysis and by binding studies with selective ligands of human embryonic kidney 293 cells (HEK293) transiently transfected with various subunit combinations. Highly sensitive receptors contain the  subunit with the above mentioned a6b2d receptors being the most sensitive ones. Furthermore, GABA does not seem to be the agonist with the highest efficacy on all GABAAR as gaboxadol displayed a higher efficacy than GABA on a6b3 and a6b3 receptors but not on a6b3g2 receptors.