Distribution of the mRNA for a metabotropic glutamate receptor, mGluR3, which is coupled to the inhibitory cAMP cascade, was examined in the central nervous system of the adult albino rat by in situ hybridization. The hybridization signals of mGluR3 were detected not only on neuronal cells but also on many glial cells throughout the brain and spinal cord. In the neuronal cells, prominent expression of mGluR3 mRNA was seen in the thalamic reticular nucleus. Moderately labeled neurons were seen in the anterior olfactory nucleus, cerebral neo- and mesocortical regions, lateral amygdaloid nucleus, ventral part of the basolateral amygdaloid nucleus, dorsal endopiriform nucleus, supraoptic nucleus, superficial layers of the superior colliculus, inferior colliculus, interpeduncular nucleus, superior olivary nuclei, and Golgi cells in the cerebellar cortex. Weakly labeled neurons were observed in the striatum, nucleus accumbens, ventral pallidum, globus pallidus, entopeduncular nucleus, lateral hypothalamic area, hypothalamic paraventricular nucleus, medial habenular nucleus, anterior pretectal nucleus, Barrington's nucleus, Nucleus O, paragenual nucleus, trigeminal sensory complex, cochlear nuclei, dorsal motor nucleus of the trigeminal nerve, dorsal cap of the inferior olive, spinal dorsal horn, and lamina X of the spinal cord. The stellate cells in the cerebellar cortex, and neurons in the deep cerebellar nuclei were also labeled weakly. The granule cell layer of the dentate gyrus, as a whole, appeared to be labeled intensely, but each of the granule cells was labeled only weakly. No significant labeling was detected in the mitral and tufted cells in the olfactory bulb, hippocampal pyramidal cells, Purkinje and granule cells in the cerebellar cortex, or somatic motoneurons. The distribution of mGluR3 mRNA in particular neurons and glial cells indicates specific roles of mGluR3 in the glutamatergic system of the central nervous system.