The immunocytochemical localization of the recently cloned metabotropic glutamate receptor 1 alpha (mGluR1 alpha) was demonstrated with a C-terminus specific antibody in rat cerebellar cortex. This antibody detects a 138-140 kDa major, and a 46 kDa minor band in membrane preparations of rat cortex and cerebellum. mGluR1 alpha immunoreactivity (mGRi) was present in Purkinje and basket cells. Purkinje cell dendritic spines and their postsynaptic membranes showed selective labelling. Presynaptic membranes, parallel fibres and glial processes were devoid of mGRi. It is suggested that the selective postsynaptic localization of this receptor at the dendritic spines of Purkinje cells serves as the morphological basis for long term depression processes in the molecular layer of the cerebellar cortex.