The hypouricemic effect of benzbromarone has been investigated in six subjects. Benzbromarone increased urate: creatinine by 371 per cent over control values at two to four hours after administration. Over a 24 hour period, the mean serum uric acid decreased from a control value of 7.8 +/- 0.8 to 4.3 +/- 0.6 mg/dl. This uricosuric effect was completely reversed by pyrazinamide, partially inhibited by acetylsalicyclic acid and sulfinpyrazone, and was not accompanied by an elevation of the creatinine clearance or an inhibition of urate binding to plasma protein. In vitro studies showed only 22 per cent inhibition of urate binding by benzbromarone five muM, a concentration which is transiently reached in man. Kinetic studies of human liver xanthine oxidase demonstrated non-competitive inhibition with variable hypoxanthine and a Ki slope of 8.5 muM. The Ki slopes for benzarone and allopurinol were 19.0 muM and 0.05 muM respectively. There was no elevation of the urinary oxypurines following benzbromarone ingestion. These observations suggest that only the renal tubular activity of benzbromarone is relevant to its hypouricemic effects in man. (J Rheumatol 2: 437-445, 1975).