Furosemide (40 mg) was administered to 18 healthy adult males as an intravenous dose, an oral solution, and in tablet form. The pharmacokinetics of intravenous furosemide were studied, determining a total body clearance rate of 117.6 +/- 41.3 ml/min and a harmonic mean half-life of 78 min. The mean absolute bioavailability determined by ratio of areas under the plasma-time curves was 64 and 71% for the solution and tablet, respectively. The mean absolute bioavailability determined by the ratio of urinary cumulative excretion data was 61 and 66% for the solution and tablet, respectively. The absolute bioavailability of furosemide determined with plasma and urine data were not significantly different. Thus, urine data alone may be used to establish bioavailability of furosemide. Inspection of plasma-time curves revealed secondary maxima in several subjects, suggesting enterohepatic cycling.