By selectively regulating solute or fluid reabsorp-tion, the kidneys play the major role in maintaining the volume and composition of extracellular fluid. Many diseases, including congestive heart failure, hepatic cir-rhosis, and Cushing’s syndrome (glucocorticoid excess), are associated with or cause significant alterations in ex-tracellular fluid balance. Diuretics inhibit renal sodium transport and thereby interfere with the normal regula-tory activity of the kidney. In some instances, adminis-tration of a diuretic drug is the primary treatment indi-cated, while in others it is one of several drugs that are used as part of a treatment regimen. In either case, an ideal diuretic would be one that caused the excretion of “extra” urine with an electrolyte composition similar to that of normal plasma. No such diuretic exists. Thus, al- though diuretic therapy provides welcome relief from pulmonary congestion, ascites, edema, and hyperten-sion, it also invites complications of organ hypoperfu-sion that may be accompanied by marked distortions of plasma composition.