It is now generally accepted that the intracellular ionized magnesium concentration ([Mg2+]i) in muscle cells is around 1 mmol l-1; in heart muscle this means that from the total some 90-95 % is bound (see McGuigan et al. 1991a). Although binding will include sequestration by intracellular organelles, a large part of the binding is by ATP in the cytosol and an equilibrium exists in the cytosol between free ATP, ionized magnesium and Mg-ATP. The extend of this equilibrium depends on the equilibrium constant of the reaction, which is a function of pH, temperature and ionic strength. This equilibrium constant is also important in the estimation of [Mg2+]i using 31P-NMR. In this method the difference between the α and β peaks of ATP is measured and from this shift and the equilibrium constant between Mg2+ and ATP in the cytosol, the [Mg2+]i can be calculated (Vink, 1993).