Guanine nucleotide-binding regulatory proteins (G proteins) mediate many receptor-coupled signal transduction events. We have localized in rat brain by in situ hybridization the mRNA for the G protein subunits--G alpha s, G alpha o, and G beta. Oligonucleotide probes were radiolabeled by a technique that resulted in a probe of defined specific activity and uniform length. mRNA species encoding G alpha s and G beta occur in high densities heterogeneously throughout the brain, especially in large neuronal cell bodies--e.g., hippocampal pyramidal cells, granule cells of the dentate gyrus, hypothalamic nuclei, and neurons of brainstem nuclei and the reticular formation. G alpha o mRNA has a more limited distribution and abundance, being detectable in the claustrum, endopiriform nucleus, habenula, hippocampal pyramidal cells, granule cells of the dentate gyrus, and cerebellar Purkinje cells.