The metabotropic glutamate receptor mGluR5 is a G-protein coupled receptor that plays a key role in release of Ca2+ from internal stores via inositol triphosphate mobilization. Western and Northern blot analyses revealed a greatly enhanced expression of mGluR5 in rats during early stages of hypothalamic development compared with the adult. This enhanced developmental expression provides an explanation for the dramatic physiological response of developing neurons to metabotropic glutamate receptor activation and supports the argument that metabotropic glutamate receptors may play an important role in hypothalamic development. During development, expression of the mGluR5 gene was reduced, not only in the hypothalamus but also in other regions of the brain. A differential decrease in mGluR5 protein was found in different brain regions with Western blot analysis. The hypothalamus showed a sixfold decrease in mGluR5 with development, whereas the cortex showed only a threefold decrease. Immunocytochemistry with an affinity-purified antibody against a peptide deduced from the cloned mGluR5 gene revealed selective expression in some regions in the adult hypothalamus. In the adult and developing (postnatal day 10) brain, immunoreactive neurons were found in the suprachiasmatic nucleus, preoptic area, lateral hypothalamus, and mammillary region, areas where the related metabotropic glutamate receptor mGluR1 is also found. In contrast, the ventromedial nucleus, an area critically involved in the regulation of food intake and metabolic balances, showed strong mGluR5 immunoreactivity but no mGluR1 immunoreactivity. Little or no mGluR5 staining was found in the neurosecretory neurons of the paraventricular, supraoptic, and arcuate nuclei. Ultrastructurally, mGluR5 was associated with the cytoplasmic face of the plasmalemma on hypothalamic dendrites, dendritic spines, and neuronal perikarya in the adult. The strongest immunoreactivity was found in patches on the membrane, sometimes associated with the postsynaptic side of synapses and sometimes associated with nonsynaptic dendritic or perikaryal membrane. Intense immunostaining was found on some astrocyte processes surrounding synaptic complexes containing asymmetrical synapses. These astrocytes would be in an ideal position to receive excitatory signals from glutamatergic axons. Unlike the punctate appearance of immunolabeling on neuronal membranes, astrocytes showed continuous staining along the plasma membrane.