Gamma-aminobutyric acid receptors (GABA(B)Rs) are G-protein coupled receptors mediating the slow-onset and prolonged synaptic actions of GABA in the central nervous system (CNS). There are two subtypes, GABA(B1)R and GABA(B2)R, that are though to form heterodimers, with the GABA(B1)R subunit essential for ligand binding and the GABA(B2)R subunit for cell surface localization. We have analyzed the distribution of GABA(B1)R and GABA(B2)R transcripts by in situ hybridization. The two transcripts were generally expressed in parallel, and the highest levels were detected in the piriform cortex, hippocampus, medial habenula, and olfactory bulb. There was moderate expression in several thalamic nuclei and the cortex area. In contrast to the distribution of GABA(B2)R mRNA, GABA(B1)R gave weak signals throughout the thalamus, piriform cortex, and field CA2 of the hippocampus, but strong signals in the septum, superior colliculus, retrosplenial cortex area, and field CA1 of the hippocampus. Thus GABA(B2)R and GABA(B1)R mRNA expression overlapped considerably, consistent with the suggestion that the two subtypes assemble to form a heterodimer. There are nevertheless differences in their expression levels that suggest at the existence of unidentified receptor subtypes.