We have identified and isolated human and rat cDNAs for a novel receptor, gb2, with 38% homology to the GABA(B) receptors gb1a and gb1b. These receptors comprise a new subfamily of seven transmembrane G protein-coupled receptors (GPCRs) that share structure and sequence similarities with the metabotropic glutamate receptors. In situ hybridization histochemistry using an antisense probe to this novel receptor mRNA shows a distribution in rat CNS nearly identical to that for the gb1 receptor, although some regions showed significant differences. Specifically, message levels for gb2 were virtually absent in the caudate/putamen, and significantly lower in the medial basal hypothalamus, septum and brainstem as compared with gb1 message levels. In contrast to gb1, gb2 mRNA was never detected in white matter suggesting that gb2 message is found exclusively in neurons. Finally, in rat brain regions showing significant overlap of message for gb1 and gb2, the transcripts are often found in the same cells. Data from our previous work showing that coexpression of gb2 with gb1 is necessary for expression of a functional receptor together with the detailed anatomical data presented here indicate that native GABA(B) receptors function as heteromeric proteins, the most abundant form being the gb1/gb2 receptor. However, the more limited distribution of gb2 receptor mRNA suggests that there are brain regions where GABA(B) receptors are composed of gb1 and as yet unidentified family members.