An assay using reversed-phase high-performance liquid chromatography with ultraviolet detection, at 278 nm, was developed to measure diclofenac in human plasma and urine at concentrations suitable for biopharmaceutical studies. Indomethacin was used as internal standard and separation was performed at 40 degrees C on a C18 Spherisorb column with acetonitrile-0.1 M sodium acetate (35:65, v/v) (pH 6.3) as mobile phase. The sample preparation is simple and rapid (extractionless), and the total run time is less than 5 min. The retention time is 2.8 min for diclofenac and 3.6 min for indomethacin. The detection limit is 0.2 microgram/ml using a 20-microliters loop.