The present study was designed to refute or confirm the postulate that the use of high dose frusemide in chronic renal failure will increase the glomerular filtration rate (GFR). 7 patients in stable chronic renal failure were admitted to hospital for 20 days. For 10 days fluid balance was maintained and no diuretics given. During the next 10 days, a gram of frusemide was given orally, with fluid and electrolyte replacement based on the 24-hour urine collections which were made throughout the 20-day period. Daily estimations of sodium, potassium, urea, creatinine and osmolality were made on plasma and urine. The excretion of water (p less than 0.01), sodium (p less than 0.01) and potassium (p less than 0.05) increased significantly during the treatment period. There was no significant change in blood pressure, weight, creatinine clearance (C/CR), clearance of urea (C/Urea), total urea and creatinine excretion, serum sodium, potassium or osmolality. The results demonstrate that a gram of frusemide, administered to patients in stable chronic renal failure, does not have any effect on intrinsic renal function. However, there was a significant increase in urine volume, as well as sodium and potassium excretion.