Furosemide was administered intravenously to 5 healthy volunteers and 15 patients with various degrees of renal failure. Two patients were given the drug orally. The plasma half-life of furosemide averaged 0.79 hr in the healthy subjects. Although most patients with kidney disease had a prolonged half-life (t1/2), up to 24.58 hr, some with advanced renal failure had an almost normal t1/2. The plasma clearance of furosemide, which in the normal subjects averaged 194 ml/min, decreased proportionally with decreasing creatinine clearance, as did the renal clearance, which in the healthy subjects averaged 95 ml/min. There was no correlation between kidney function and the apparent volume of distribution or of nonrenal clearance. One patient was given 35S-labeled furosemide intravenously. Although the furosemide plasma t1/2 was essentially normal, the elimination rate of metabolites was decreased. Unlike that of healthy subjects, the main route of excretion of label was in the feces.