Clofibric acid disposition was studied in four healthy men after 1 wk of clofibrate ingestion (500 mg orally every 12 hr) with and without probenecid (500 mg orally every 6 hr). Mean (+/- SD) free clofibric acid plasma concentration in the four subjects over a dosage interval at steady state was 2.5 +/- 0.03 mg/1 before and 9.05 +/- 1.09 mg/1 after the probenecid. Probenecid reached an average plasma concentration of 71.3 mg/1. No clofibric acid glucuronide was detected in plasma during either treatment. The fractions of the dose recovered in urine as clofibric acid, clofibric acid glucuronide, and clofibric acid liberated after acid hydrolysis were not altered by probenecid. These data suggest that probenecid causes a reduction in renal and metabolic clearance of clofibric acid, probably as a result of inhibition of the conjugation of clofibric acid with glucuronide.