It is well known that calcium acts as a vital intracellular second messenger that governs a large array of cellular processes. However, the molecular identification of a receptor for extracellular Ca2+, the extracellular calcium-sensing receptor, has opened up the possibility that Ca²⁺might also function as a messenger outside the cell. In animals, the Ca²⁺ sensor is the well-characterized extracellular-Ca²⁺- sensing receptor (CaR), a G- protein -coupled receptor originally isolated from the parathyroid gland. In addition, other receptors, channels and membrane proteins are all sensitive to external Ca2+ fluctuations. Recently, Han et al have cloned a receptor protein for extracellular calcium in Arabidopsis, which plays a key role in Ca²⁺-induced stomatal closing. Thus, the cloning of these receptors has prompted the consideration of Ca²⁺ also functioning as a ‘first messenger’ in animals and plants.