Azapropazone is an anti-inflammatory agent with reported uricosuric properties. The aim of the present study was to extend these observations, by examining the dose-response and to compare the uricosuric effect of azapropazone with that of probenecid. Patients were given varying doses of azapropazone from 900-2400 mg daily for 4-day periods at separated intervals. Plasma uric acid levels were measured before and at the end of each treatment period. Three other patients maintained on low purine diets were given a 4-day course of 1200 mg azapropazone daily followed at an interval by a 4-day period of probenecid 1 g daily. Plasma uric acid levels and 24 h urinary uric acid excretion were compared. The mean fall in plasma uric acid level after four days of 900 mg azapropazone daily was 31.4% (n = 9) compared with 33.9% (n = 12) on 1200 mg daily; 42.3% (n = 10) on 1800 mg daily; and 46% (n = 6) on 2400 mg daily, indicating a graded dosage response. In the three patients on low purine diets the falls in plasma uric acid levels on probenecid 1 g daily were 50.5%, 46% and 29% compared with 33.5%, 32% and 20% respectively on azapropazone 1200 mg daily. Similarly the total amount of uric acid excreted in the urine by each patient during the 4-day period on probenecid 1 g daily was 14.01; 13.03 and 8.97 mmol compared with 23.53, 10.9 and 7.69 mmol on azapropazone 1200 mg daily.(ABSTRACT TRUNCATED AT 250 WORDS)