A high-performance liquid chromatographic method for the quantitation of nimesulide in human plasma is presented. The method is based on protein precipitation with methanol and reversed-phase chromatography with spectrophotometric detection at 404 nm. The separation was performed on a Nucleosil 120-5 C18, 50 x 4-mm I.D. column and the mobile phase consisted of acetonitrile-methanol-15 mM potassium dihydrogenphosphate buffer, pH 7.3 (30:5:65, v/v). Only 250 microl of plasma are used for sample preparation and no internal standard is necessary. The limit of quantitation is 80 ng/ml and the calibration curve is linear up to 10,000 ng/ml. More than 20 samples can be analysed within 1 h. Within-day and between-day precision expressed by relative standard deviation is less than 5% and inaccuracy does not exceed 8%. The assay was used for pharmacokinetic studies.