Group I metabotropic glutamate receptors (mGluR1/5) are important to synaptic circuitry formation during development and to forms of activity-dependent synaptic plasticity. Dysregulation of mGluR1/5 signaling is implicated in some disorders of neurodevelopment, including fragile X syndrome, the most common inherited form of intellectual disabilities and leading cause of autism. Site(s) in the intracellular loops of mGluR1/5 directly bind caveolin-1, an adaptor protein that associates with membrane rafts. Caveolin-1 is the main coat component of caveolae and organizes macromolecular signaling complexes with effector proteins and membrane receptors. We report that long-term depression (LTD) elicited by a single application of the group I mGluR selective agonist ( RS )-3,5-dihydroxyphenylglycine (DHPG) was markedly attenuated at Schaffer collateral-CA1 synapses of mice lacking caveolin-1 ( Cav1 −/− ), as assessed by field recording. In contrast, multiple applications of DHPG produced LTD comparable to that in WT mice. Passive membrane properties, basal glutamatergic transmission and NMDA receptor (NMDAR)-dependent LTD were unaltered. The remaining LTD was reduced by anisomycin, an inhibitor of protein synthesis, by U0126, an inhibitor of MEK1/2 kinases, and by rapamycin, an inhibitor of mammalian target of rapamycin (mTOR), suggesting mediation by the same mechanisms as in WT. mGluR1/5-dependent activation (phosphorylation) of MEK and extracellular signal-regulated kinase (ERK1/2) was altered in Cav1 −/− mice; basal phosphorylation was increased, but a single application of DHPG had no further effect, and after DHPG, phosphorylation was similar in WT and Cav1 −/− mice. Taken together, our findings suggest that caveolin-1 is required for normal coupling of mGluR1/5 to downstream signaling cascades and induction of mGluR-LTD.