RGS proteins (regulators of G protein signaling) are a newly identified group of proteins that negatively regulate G proteins. Some, including RGS4, have been shown to act as GTPase-activating proteins for several members of the Gi alpha subunits and the Gq alpha subunit. However, the signaling pathways of G protein-coupled receptors regulated by RGS4 are not well understood. We isolated the mouse RGS4 cDNA to investigate the roles of RGS4 protein in mouse brain. Expression of mouse RGS4 in yeast stimulated adaptation to mating pheromone, suggesting that it negatively regulates that G protein-mediated signaling pathway. Northern blot analysis of various mouse tissues revealed that RGS4 is expressed at high levels in brain, moderately low levels in heart, and very low levels in lung, liver, and skeletal muscle. In situ hybridization of mouse brain showed RGS4 mRNA mainly in the cerebral cortex, hippocampus, anterior olfactory nucleus, piriform cortex, olfactory tubercle, caudate-putamen, nucleus accumbens, islands of Calleja, substantia nigra, amygdala, the granular layer of cerebellum, middle cerebellar peduncle, and perifacial zone. These results suggest that RGS4 is expressed in various types of cells in the brain and may regulate multiple G protein-mediated signaling pathways.