The postnatal expression of metabotropic glutamate receptors was studied in rat brain by in situ hybridization and autoradiographic binding techniques. The messenger RNAs encoding five metabotropic glutamate receptor subtypes named mGluR1-5 had distinct regional and temporal expression profiles. mGluR1, mGluR2 and mGluR4 messenger RNA expression was low at birth and increased during postnatal development. In contrast, mGluR3 and mGluR5 were highly expressed at birth and decreased during maturation to adult levels of expression. [3H]Glutamate binding competition studies in developing brain disclosed the presence of two types of binding sites with the pharmacological properties of metabotropic glutamate receptors, having high (metabotropic type-1 binding sites; K1 = 8 nM) and low affinity (metabotropic type-2 binding sites; K1 = 50 μM) for quisqualic acid, as in adult rat brain. The densities of metabotropic binding sites changed during development in a complex, regionally specific fashion. Metabotropic type-1 binding sites were present at low levels at birth and gradually increased during the second postnatal week. In the striatum, globus pallidus and cerebellar granule layer, the increase in density of metabotropic type-1 binding sites was transient but persisted in the cerebellar molecular layer. In contrast, metabotropic type-2 binding sites were present at high densities in most regions in the first postnatal week and decreased during the second and third week, particularly in the thalamic reticular nucleus and globus pallidus. Only in the external cortex did both metabotropic type-1 and metabotropic type-2 binding sites increase during development. A striking correspondence between the temporal pattern of expression of specific metabotropic glutamate receptor transcripts and metabotropic binding sites was observed in the reticular nucleus of the thalamus (mGluR3; metabotropic type-2 binding sites) and cerebellum (mGluR1; metabotropic type-1 binding sites) suggesting early translation of these metabotropic glutamate receptor messenger RNAs into receptor proteins. In other regions the relationship between messenger RNA expression and binding sites was less direct: comparison between expression of metabotropic glutamate receptor messenger RNA and binding sites suggests both a pre- and postsynaptic location of some receptor subtypes. These data imply a functional role of mGluR3 and mGluR5 during synaptogenesis and maintenance of adult synapses and of mGluR1, mGluR2 and mGluR4 in mature synaptic transmission. © 1994.