Following cell surface receptor activation, the α subunit of the Gq subclass of GTP‐binding proteins activates the phosphoinositide signalling pathway. Here we examined the expression and localization of Gq protein α subunits in the adult mouse brain by in situ hybridization and immunohistochemistry. Of the four members of the Gq protein α subunits, Gαq and Gα11 were transcribed predominantly in the brain. The highest transcriptional level of Gαq was observed in cerebellar Purkinje cells (PCs) and hippocampal pyramidal cells, while that of Gα11 was noted in hippocampal pyramidal cells. Antibody against the C‐terminal peptide common to Gαq and Gα11 strongly labelled the cerebellar molecular layer and hippocampal neuropil layers. In these regions, immunogold preferentially labelled the cytoplasmic face of postsynaptic cell membrane of PCs and pyramidal cells. Immunoparticles were distributed along the extra‐junctional cell membrane of spines, dendrites and somata, but were almost excluded from the junctional membrane. By double immunofluorescence, Gαq/Gα11 was extensively colocalized with metabotropic glutamate receptor mGluR1α in dendritic spines of PCs and with mGluR5 in those of hippocampal pyramidal cells. Together with concentrated localization of mGluR1α and mGluR5 in a peri‐junctional annulus on PC and pyramidal cell synapses (Baude et al. 1993, Neuron, 11, 771–787; Luján et al. 1996, Eur. J. Neurosci., 8, 1488–1500), the present molecular‐anatomical findings suggest that peri‐junctional stimulation of the group I metabotropic glutamate receptors is mediated by Gαq and/or Gα11, leading to the activation of the intracellular effector, phospholipase Cβ.