Recent developments in our knowledge of cellular calcium metabolism are reviewed. Changes in the Ca2+ concentration in the cell cytosol are a nearly universal means of coupling stimulus with response in both neural and endocrine cells. In particular, Ca2+ serves as an important intracellular messenger in the regulation of cardiac and smooth muscle function, the regulation of aldosterone secretion by K+ and angiotensin II, and the secretion of renin. Several points about the messenger role of Ca2+ are emphasized: small changes in Ca2+ concentration bring about large changes in cell response; the regulation of calcium fluxes across the plasma membrane are critical to proper cell function; and excessive calcium is a cellular toxin. An attempt is made to integrate our knowledge of the function and properties of intracellular Ca2+ receptor proteins with our knowledge of cellular calcium homeostasis, and relate this to the pathogenesis of hypertension.