In 115 randomized patients with left and/or right ventricular failure, the effect of the new diuretic Etozolin (800 mg p.o.) (n = 55) is compared with that of the loop diuretic Furosemide (80 mg p.o.) (n = 60). RESULTS 1. The increased diuresis after Etozolin remains constant during the entire trial period. Furosemid initially induces a more intense diuresis. 2. Analysis of the action profile shows that the higher daily urinary output following Furosemide is due to a more intense diuresis in the first fractions of the day. The action of Etozolin is more constant and lasts into the evening. 3. Both substances reduce body weight to the same extent. 4. Heart rate and arterial blood pressure decline significantly during trial. 5. Etozolin induces lesser electrolyte elimination than Furosemide in the initial phase of the trial. Potassium elimination values, in particular, remain below Na+ and Cl- elimination values for both substances. 6. During the trial period both substances had no significant effect on blood and liver values, serum electrolytes, creatinine, urea and uric acid. Etozolin may be classified as a diuretic similar to a thiazide derivative in its 24-h profile but behaves like a loop diuretic in its diuretic effect.