35S-furosemide was administered orally (n=7) or i.v. (n=2) to healthy subjects. The average gastrointestinal uptake estimated by comparison of the urinary recovery of label and the areas under the plasma curves after the two routes of administration was 65%. The half life of radioactivity in the plasma after oral 35S-furosemide was 90 +/- 17 min (estimated on the slope between 2 and 6 h); the corresponding figure after 35S-furosemide i.v. was 47-53 min (slope 0.5-4 h). There was probably a slower phase after 4-6 h. Fractionation of labelled material in urine from two subjects demonstrated that approximately two thirds of the label recovered at 24 h had the same chromatographic properties as furosemide. A major part of the metabolite(s) was probably furosemide glucuronide. There was no evidence that 4-chloro-5-sulfamoylanthranilic acid was formed in man. The total urinary recovery of label (5-7 d) after oral and intravenous administration was 55.1 +/- 3.2 (mean +/- SD) and 82-84%, respectively. After 35S-furosemide i.v., 6-9% of the label was recovered in faeces, and it could not be accounted for solely by biliary excretion of furosemide.