The cellular and subcellular distribution of the GluR1 subunit of the AMPA-type excitatory amino acid receptor was determined in the cerebellar cortex of rat using immunocytochemistry. Two polyclonal antibodies were raised against the N- and C-terminal regions of the subunit. They both labeled a band in immunoblots of rat cerebellar membranes with a molecular weight corresponding to that predicted for this subunit of 105 kDa molecular mass. In light microscopy the distribution of immunoreactivity for the two antibodies was very similar. The molecular layer was strongly immunoreactive whereas no labeling was observed in the granular layer. Electron microscopy revealed that the antibody raised against the N-terminal part of the subunit recognizes an extracellular epitope(s), whereas the antibody against the C-terminal part recognizes an intracellular epitope(s) along the plasma membrane. In Bergmann glial cells the endoplasmic reticulum, Golgi apparatus, and multivesicular bodies were labeled, presumably demonstrating sites of synthesis and degradation for the GluR1 subunit, respectively. Immunoreactivity was associated with Bergmann glial processes surrounding Purkinje cell dendrites, spines, and the glutamate-releasing axon terminals of the parallel and climbing fibers. This suggests that the neurotransmitter glutamate and the AMPA- type glutamate receptors are involved in neuronal/glial communication. The GluR1 subunit was also found at glial membranes in contact with other glial cells. Purkinje cells showed immunoreactivity in the endoplasmic reticulum and multivesicular bodies. No immunoreaction was detected in basket and stellate cells. Immunoreactivity was observed at type 1 synaptic junctions, including the synaptic cleft. These synaptic junctions were between spines, often originating from Purkinje cell dendrites, and parallel or climbing fiber terminals. Our results demonstrate that the GluR1 subunit of the AMPA-type ionotropic excitatory amino acid receptor is present at both parallel and climbing fiber synapses, which are surrounded by glial processes containing the same receptor subunit.