Muzolimine (240 mg) was administered orally to 24 patients with chronic renal failure. In five patients with creatinine clearance ranging between 10 and 30 ml/min and five others with creatinine clearance lower than 10 ml/min, urinary output was measured in periods of 24 hours before, and 0 to 4, 4 to 10, 10 to 24, 24 to 48, 48 to 72 hours after administration of the drug. The amounts of sodium, chloride, potassium and urea excreted in each sample were determined. Our results show that the diuretic activity of muzolimine 240 mg in these patients is excellent, especially in the first four hours after its administration. Pharmacokinetic data observed in advanced renal failure (creatinine clearance between 10 and 30 ml/min) are similar to those observed in healthy subjects or patients with normal renal function. However, attention should be paid to possible accumulation of the drug in terminal renal failure (creatinine clearance lower than 10 ml/min) because a significant increase of the terminal half-life of the drug is observed in these patients. Dialysibility of the drug in haemodialysis and during chronic ambulatory peritoneal dialysis is poor.