Previous studies have described a pharmacokinetic interaction between probenecid, a uricosuric drug, and oxipurinol, the major metabolite of allopurinol. In single dose studies, no interaction was found to occur between benzbromarone, another uricosuric agent, and oxipurinol. A cross over study was conducted in 12 volunteers to compare the kinetics of allopurinol and oxipurinol following treatment for 7 days with allopurinol alone or combined with benzbromarone 20 or 100 mg. The pharmacokinetic parameters of allopurinol were not modified by the uricosuric therapy, but those of oxipurinol were markedly altered by concurrent administration even of the lower dose of benzbromarone; the average plasma level fell by 30% and the renal elimination rate was increased by 50%. A parallel increase in the renal elimination rate of uric acid was observed (significant only with the higher dose of benzbromarone) and a positive linear correlation between the fractional excretion of uric acid and that of oxipurinol was established.