The authors describe the effect of a single i.v. dose of 20 mg furosemide on the excretion of oxalic acid, vitamin C and vitamin B6 in a control group and in patients with chronic renal failure without dialyzation treatment. An increased urinary excretion of oxalic acid, vitamin C and vitamin B6 was found during the first three hours after furosemide administration in both groups. The effect of furosemide persisted for six hours in patients with chronic renal failure without dialyzation treatment. The authors described a new hitherto unknown positive side-effect of furosemide, i.e. enhanced urinary oxalic acid excretion in the control group and in patients with chronic renal failure without dialyzation treatment and a negative side-effect of furosemide, i.e. increased urinary vitamin B6 excretion in both examined groups. With regard to the assembled results the authors recommend in addition to monitoring of vitamin C also monitoring of vitamin B6 in plasma during long-term administration of large doses of furosemide to patients with chronic renal failure as deficiency of these vitamins could develop.