Localization of a metabotropic glutamate receptor, mGluR4a, was immunohistochemically examined in the rat cerebellum with an antibody, which was produced by using a synthetic peptide corresponding to a C-terminal sequence of rat mGluR4a. Marked mGluR4a-like immunoreactivity (mGluR4a-LI) was seen in neuropil of the molecular layer of the cerebellar cortex. Electron microscopically, mGluR4a-LI was observed in many axon terminals in the molecular layer. These axon terminals showing mGluR4a-LI were filled with round synaptic vesicles and were in asymmetric synaptic contacts most frequently with dendritic spines. The results indicate that mGluR4a are located presynaptically in the parallel fibers arising from the granule cells in the cerebellar cortex.