Bumetanide, a new diuretic exerting its major effect on the ascending limb of the loop of Henle, was evaluated in 20 patients with congestive heart failure. Dosage ranged from 1 mg to 3 mg daily depending on the patient's condition. The results after 3 and 8 days' treatment showed that bumetanide caused a significant diuresis, an increased excretion of sodium, potassium and chloride, and a comparable fall in the serum levels of these electrolytes. Changes in electrolyte levels were directly related to the dose of the drug. The resultant hypochloraemia was accompanied by a slight metabolic alkalosis. A comparative crossover study between placebo, bumetanide and frusemide using equipotent doses was performed in 10 patients. Both drugs had a similar effect upon water excretion and the serum and urinary electrolytes.