Furosemide kinetics were studied in 4 normal subjects after single intravenous injections (1 mg/kg). One experiment was done after pretreatment with probenecid. The apparent volume of furosemide distribution was unchanged after probenecid (10.9 L). The mean plasma clearance fell from 155 to 85 ml/min and the mean plasma t1/2 rose from 36 to 61 min. Renal clearance of furosemide fell below 50% of control after probenecid, but the kidney remained the main route of its excretion (75% of the dose appeared in the urine). In another experiment in 4 subjects an infusion of furosemide was sustained following a loading dose to maintain a constant plasma level. After a control period, probenecid was given orally. This resulted in a decrease in renal excretion of furosemide with a simultaneous rise in its plasma concentration. Despite the rising plasma furosemide concentration, however, there was a diminution in both urine flow and the excreted fraction of filtered sodium, which suggested some reduction of diuretic action. In doses commonly used, probenecid reduces renal elimination of furosemide in man with only a mild impairment of its diuretic activity. This suggests that furosemide is eliminated predominantly by way of proximal tubular secretion and that tubular rather than plasma concentration is the main determinant of its diuretic effect.