MAINTENANCE of a stable internal environment within complex organisms requires specialized cells that sense changes in the extracellular concentration of specific ions (such as Ca²⁺). Although the molecular nature of such ion sensors is unknown, parathyroid cells possess a cell surface Ca²⁺-sensing mechanism that also rec-ognizes trivalent and polyvalent cations (such as neomycin) and couples by changes in phosphoinositide turnover and cytosolic Ca²⁺ to regulation of parathyroid hormone secretion^1–4. The latter restores normocalcaemia by acting on kidney and bone². We now report the cloning of complementary DNA encoding an extracellular Ca²⁺ -sensing receptor from bovine parathyroid with pharmacological and functional properties nearly identical to those of the native receptor. The novel ∼120K receptor shares limited similarity with the metabotropic glutamate receptors⁵ and features a large extracellular domain, containing clusters of acidic aminoacid residues possibly involved in calcium binding, coupled to a seven-membrane-spanning domain like those in the G-protein-coupled receptor superfamily.