We tested the hypothesis that the regional, cellular, and synaptic localizations of the glutamate receptor 1 (GluR 1) subunit of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor are regulated developmentally in rat brain. By immunoblotting, GluR1 was first detected in whole brain at embryonic day E15.5, and levels increased progressively during late embryonic (E20) and early postnatal (P2-P11) days. Regionally, GluR1 increased in cerebral cortex but decreased in striatum with postnatal maturation. These changes occurred in the presence of increased presynaptic maturation, as determined by synaptophysin detection. By immunocytochemistry, distinct cellular populations showed different temporal profiles of GluR1 expression during postnatal maturation. The neocortex and hippocampus showed a progressive maturation-related enrichment of GluR1, whereas the striatum showed a gradual reduction in GluR1 during maturation. In cerebellum, GluR1 protein was expressed transiently at restricted times postnatally by granule cells (P0-P11) and Purkinje cells (P13-P19), but by P21 and thereafter these neurons had sparse GluR1 immunoreactivity. By immunoelectron microscopy. GluR1 was found in neurites, specifically in both dendritic and axon terminal components of developing synapses. GluR1 was clustered at the plasma membrane of apparent growth cone appositions, neuronal cell bodies, and dendrites of developing neurons. The presence of GluR1 at presynaptic sites dissipated with synaptic maturation, as GluR1 became confined to the somatodendritic compartment as maturation progressed. We conclude that the regional expression as well as the cellular and synaptic localizations of the GluR1 are developmentally regulated and are different in immature and mature brain. Differences in glutamate receptor expression and synaptic localization in immature and mature brain may be relevant to the phenomenon that the perinatal and adult brain differ in their regional vulnerability to hypoxia-ischemia and excitotoxicity.