Studies of g-aminobutyric acid (GABA) B receptor function in heterologous cell systems have suggested that expression of two distinct seven transmembrane G-protein coupled receptor subunits is necessary for receptor activation and signal transduction. Some results suggest that both receptor proteins must be inserted into the plasma membrane to create heterodimers; however, it is possible that subunit monomers or homodimers are functional in cells which constitutively express GABA B receptors. A new pituitary intermediate lobe melanotrope cell clone (mIL tsA58) has been isolated which constitutively expresses GABA B , D 2 and corticotrophin releasing factor receptors. Here, we report on characterization of the GABA B receptors. Solution hybridization-nuclease protection assays reveal the presence of GABA B(1) and GABA B(2) transcripts. Western blots show GABA B(1a) and one of two GABA B (2)