Free intracellular Mg2+ ([Mg2+](i)) can potentially integrate the signals from hormones, cellular metabolism and organismal ion homeostasis and affect the activities of ion channel and other effecters. Interest in [Mg2+](i) has been heightened by recent reports that small changes in [Mg2+](i) in the physiological range can significantly modulate Important cellular functions. In addition, a variety of new evidence shows that [Mg2+](i) instantaneously changes following stimulation with various biologically active substances. These observations suggest that [Mg2+](i) may act as a second messenger.