Because of the significance of potassium in the practice of medicine, it is important to consider factors that influence the integrity of intracellular and extracellular K. Clinicians today are skilled in managing hypokalemia as well as hyperkalemia. However, physicians may be less aware of the role of magnesium in K homeostasis. Experimental observations to date support the view that Mg and K metabolism are closely linked. Dietary Mg depletion in the rat is accompanied by muscle K depletion, which occurs despite provision of K.1,2 Concurrent K and Mg depletion results in a greater loss of muscle K than with K depletion alone.3 In K- and Mg-deficient rats, muscle-cell K repletion is not optimized unless both K and Mg deficits are simultaneously corrected.4 Thus, Mg appears to be important not only in maintenance of cell K but also in conservation and repletion of cell K.5 These experimental