To investigate the role of M1 muscarininc acetylcholine receptors (m1 receptors) in metabotropic glutamate receptor (mGluR)-mediated long-term depression (LTD), we produced mouse lines in which deletion of the m1 gene is restricted to the forebrain (FB–m1KO) or hippocampal CA3 pyramidal neurons (CA3–m1KO). Stimulation in FB–m1KO hippocampal slices resulted in excitatory postsynaptic potentials and long-term synaptic plasticity (long-term potentiation and LTD) similar to controls. The mice were deficient in ( S )-3,5-dihydroxyphenylglycine hydrate (DHPG)-induced mGluR LTD, which correlated with a presynaptic increase in the release of neurotransmitters. Protein kinase C (PKC) activity, which is downstream from both mGluRs and m1 receptors, was reduced in CA3 but not in CA1. The presynaptic requirement of m1 receptors was confirmed by the lack of DHPG-induced mGluR LTD in the CA1 of slices from CA3–m1KO mice. mGluR LTD was rescued by stimulating PKC activity pharmacologically in CA3–m1KO mice. These data confirm a role for PKC activation in presynaptic induction of mGluR LTD and distinguish between the roles of mGluRs and m1 receptors.