Two patients had oliguric acute renal failure in which there was no response to intravenously administered mannitol. In both cases, diuresis resulted after simultaneous administration of mannitol and ethacrynic acid intravenously. Dialysis treatment was unnecessary. The production of a greater volume of urine decreases the tendency toward excess total body water and sodium, and extracellular potassium, and simplifies management. A therapeutic trial with both these drugs may be indicated in the oliguric phase of acute renal failure.