In clinical usage the terms hypokaliemia and hypopotassemia are taken to indicate a lowering in the concentrations of this ion in serum or plasma below the usual range present in health, even though the words themselves refer to levels in blood. The lower limits of these sets of values are 3.3 mEq. per liter in growing children, in young adults, and in old men and women and above 4 mEq. per liter in newborn infants.* The fact that all of the potassium in serum is ultrafiltrable suggests that it is present entirely in the ionized form.1 Hence lowered serum levels of this electrolyte can be taken to reflect comparable changes in the potassium concentration of the extracellular fluid as a whole, i. e., in the interstitial spaces as well as in the plasma. Such lowered levels of extracellular potassium may be present without any change in the total amount