Metabotropic glutamate receptors (mGluRs) coupled to phosphoinositide hydrolysis desensitize in response to prolonged or repeated agonist exposure, and evidence suggests that this involves activation of protein kinase C (PKC). The present studies were undertaken to determine if cloned mGluR5 undergoes similar PKC-mediated desensitization and to investigate the molecular mechanism underlying PKC-induced desensitization. In Xenopus oocytes, both mGluR5a and mGluR5b showed pronounced desensitization in response to a brief activation by glutamate. Pharmacological studies clearly suggest that this desensitization requires PKC-mediated phosphorylation. Analysis of PKC consensus phosphorylation site mutants suggests that PKC phosphorylates mGluR5 at multiple sites to induce a relatively rapid form of desensitization. Because mGluRs play important roles in synaptic plasticity and in excitotoxicity, this desensitization may be involved in the dynamic regulation of these processes.