The metabotropic glutamate receptors (mGluRs) have been predicted to have a classical seven transmembrane domain structure similar to that seen for members of the G-protein-coupled receptor (GPCR) superfamily. However, the mGluRs (and other members of the family C GPCRs) show no sequence homology to the rhodopsin-like GPCRs, for which this seven transmembrane domain structure has been experimentally confirmed. Furthermore, several transmembrane domain prediction algorithms suggest that the mGluRs have a topology that is distinct from these receptors. In the present study, we set out to test whether mGluRs has seven true transmembrane domains. Using a variety of approaches in both prokaryotic and eukaryotic systems, our data provide stong support for the proposed seven transmembrane domain model of mGluR5. We propose that this membrane topology can be extended to all members of the family C GPCRs.