STIMULATION of two metabotropic glutamate-receptor subtypes, mGluR1 and mGluR5, triggers the release of Ca2+ from intracellular stores through the inositol-(1,4,5)trisphosphate (InsP(3)) pathway(1-3). Here rye report that glutamate induces single-peaked intracellular Ca2+ mobilization in mGluR1 alpha-transfected cells but elicits Ca2+ oscillations in mGluR5a-transfected cells, The response patterns of the intracellular Ca2+ increase depend upon the identity of a single amino acid, aspartate (at position 854) or threonine (at position 840), located within the G-protein-interacting domains of mGluR1 alpha and mGluR5a, respectively, Pharmacological and peptide mapping analyses indicated that phosphorylation of the threonine residue at position 840 of mGluR5a by protein kinase C (PKC) is responsible for the generation of Ca2+ oscillations in mGluR5a-expressing cells, To our knowledge this is the first evidence that PKC phosphorylation of G-protein-coupled receptors is important in producing oscillations in intracellular Ca2+ signalling.