The electrolyte and water losses in 75 instances of diuresis following Mercuhydrin were studied in 17 edematous patients. The loss of potassium was appreciable following exhibition of the drug to patients who retained their dietary sodium almost completely. Occurrence of such clinical symptoms as weakness, nausea, and ventricular premature contractions might be attributed to this loss of potassium. The fluid lost by mercurial diuresis was isotonic with body fluids. The chloride concentration was fairly constant at 151 mEq. per liter. The concentrations of sodium and potassium were more variable, the average concentrations being 97 and 35 mEq. per liter, respectively. Loss of water and electrolyte in these concentrations leaves the patient in relative alkalosis, with a loss of intracellular potassium and a gain in intracellular sodium.