It has become increasingly evident that disturbances in electrolyte and water metabolism, sometimes occurring spontaneously but usually the result of therapy, may often be of critical importance in determining the prognosis and continued response to therapy in congestive heart failure. The following discussion briefly outlines the various iatrogenic and spontaneous disorders of electrolyte metabolism that have been observed in patients with edema of cardiac origin. The emphasis is on pathogenesis, diagnosis, and clinical management. HYPOCHLOREMIC ALKALOSIS DUE TO MERCURIALS1 The effect of a diuretic on the concentrations of serum electrolytes depends on the quantities of electrolytes eliminated with the edema fluid. In the majority of patients given mercurials the eliminated fluid contains more chloride than sodium, the difference between these two usually being made up by potassium or ammonium, or both. This excessive loss of chloride with either of the latter two ions produces alkalosis in the extracellular fluid