The neurotransmitter g-aminobutyric acid (GABA) exerts its effects through an ion channel, GABAA, and a GPCR, GABAB. Functional GABAB is a heterodimer composed of the GABAB1 and GABAB2 subunits, which share 35% sequence identity and belong to the class 3 family of GPCRs. The GABAB1 subunit, which exists as splice variants GABAB1a and GABAB1b, binds directly to GABA and is required for agonist activation. The GABAB2 and GABAB1 subunits associate by formation of a coiled coil by their C-terminal tails; this association masks an ER retention sequence in GABAB1 to permit export from the ER and trafficking to the cell surface. In addition to its chaperone function, GABAB2 is the component that couples to Gi to reduce intracellular cAMP. Agonists of GABAB, such as baclofen, are used clinically for treatment of muscle spasticity, migraine headache and musculoskeletal pain (Bowery et al., 2002). Cloned human GABAB1b/GABAB2 receptor-expressing ChemiScreen cells were constructed by stable transfection of Chem-1 cells with GABAB1b/GABAB2 which supports high levels of recombinant GABAB1b/GABAB2 expression on the cell surface and contains high levels of the promiscuous G protein Gα15 to couple the receptor to the calcium signaling pathway. These stability-tested cells are ready for fluorescence-based assays for agonists, antagonists and modulators at the GABAB receptor.