The distribution of gamma-aminobutyric acid (GABA) receptor subtype B1 (GABA(B1)) mRNA-containing cells in the brain of adult rats was determined with in situ hybridization histochemistry. The vast majority of neurons expressed GABA(B1) receptor mRNA. However, there were nuclei of relative high density, and, in some nuclei, the majority of neurons did not express detectable levels of GABA(B1) receptor transcripts. Areas where the majority of neurons expressed a high density of mRNA included the medial habenula; the septohippocampal, periventricular, suprachiasmatic, and supraoptic nuclei; Purkinje cells in the cerebellum; and pyramidal and granule cells of the hippocampus and dentate gyrus, respectively. Also, brainstem nuclei containing monoaminergic neurons and neurons in the thalamic motor nuclei contained relatively high levels of expression. mRNA was low or absent in neuronal populations in regions with well-developed cytoarchitecture, such as the stratum radiatum and stratum oriens of the hippocampus, the molecular and granular layers of the cerebellum, and the molecular layer of the cortex. Low expression was observed also in many extrapyramidal nuclei, such as the globus and ventral pallidum and the substantia nigra, pars reticulata. Expression also was low in the reticular thalamic nucleus and zona incerta. Neurons lacking detectable GABA(B1) receptor mRNA were generally in nuclei that contained largely GABAergic neurons.