The urinary excretion of six ions (sodium, chloride, potassium, ammonium, bicarbonate, and phosphate) was studied by a method of fractional urine collections in a series of subjects who were receiving a diet constant with respect to water, sodium, and calories. The effects of five diuretics of different types were then compared. Each gave a distinctive pattern of electrolyte excretion. They differed with respect to potency when given in clinically tolerated doses to patients in comparable states of edema. Diuretics of the aminouracil and the carbonic anhydrase inhibitor groups lost in effectiveness after the first or second days of therapy, in spite of continued excess of sodium and water in the body; they appeared to be unsuited for use over long periods to maintain an edema-free state. Chlormerodrin and chlorothiazide, representing two different types of diuretics, were found suitable for administration by mouth in all classes of edema. With respect to economy and ease of administration, chlorothiazide had the advantage.