Hypomagnesemia and hypokalemia are both common electrolyte disturbances in the intensive care unit (ICU) setting. Magnesium and potassium have important interactions to consider when treating deficiencies of either ion. Magnesium is intimately involved in a number of biochemical reactions in the body, particularly those involving adenosine triphosphate (ATP). The ionized form of magnesium in the serum is metabolically active but represents only a small component of the total-body magnesium. Furthermore, the relationship between the ionized and serum concentrations, and the serum and intracellular (cytoplasmic) concentrations are not clear. Therefore, total serum concentrations may not be representative of active ion.