The ionic requirements for ligand binding to metabotropic glutamate receptors were carried out on secreted truncated receptors containing only the extracellular ligand binding domains of the receptors. The influence of ions on agonist binding was examined in mGluR1, mGluR3, and mGluR4 representing Group I, II, and III metabotropic glutamate receptors, respectively. [(3)H]Quisqualic acid binding to mGluR1 required the presence of calcium (or magnesium) ions but not sodium or chloride ions while [(3)H]DCG-IV binding to mGluR3 was dependent upon both cations and anions. [(3)H]L-AP4 binding to mGluR4 required chloride ions but not monovalent or divalent cations. The EC(50) for chloride facilitation of L-AP4 binding to mGluR4 was 63mM; this value is approximately one-half of the normal resting extracellular chloride concentration. These results demonstrate that metabotropic glutamate receptor subtypes require different complements of ions for ligand binding and suggest that natural physiological fluctuations in synaptic ion concentrations may regulate receptor binding and activation.