The abundance of magnesium (Mg2+) within mammalian cells is consistent with its relevant role in regulating tissue and cell functions. At the last count, more than three hundred and fifty enzymes, aside from metabolic cycles, appear to require and be regulated by concentrations of Mg2+ that are well within the physiological range observed in tissues and cells. The absence of detectable major changes in cellular free [Mg2+], and the extremely slow turn-over of the cation across the cell plasma membrane under quiescent condition has supported for more than three decades the assumption that cellular Mg2+ content is kept constant at the level necessary for enzyme and channel function, and that its concentration does not require drastic and rapid changes to form complex with ATP and other phosphonucleotides. In the last decade, a large body of new experimental observations has significantly reverted this way of thinking. Compelling evidence now suggests that large fluxes of Mg2+ can cross the cell plasma membrane in either direction following a variety of hormonal and non-hormonal stimuli, resulting in major changes in total and, to a lesser extent, free Mg2+ content within tissues, and in a marked variation in the opposite direction of circulating Mg2+ level. The present review will attempt to update our knowledge in this area and provide some insights on how changes in cellular Mg2+ content can result in a modification of the activity rate for several cellular enzymes.