OBJECTIVE: To determine the concentration of nimesulide in human plasma. METHOD: After addition of the internal standard (IS; tolbutamide), 0.5 ml of methanol and 0.3 ml of hydrochloric acid (1 mol·L^-1) were added to 0.5 ml plasma. The nimesulide was extracted into 2 ml of toluene. The organic layer was evaporated to dryness under a stream of nitrogen, and the residue redissovled in 150 μl of mobile phase for HPLC injection. The drug and IS were chromatographed on C₁₈column with a mobile phase consisting of 0.05 mol·L^-1 potassium dihydrogen phosphate solution (pH 5.02): methanol (55∶45) and detected at 230 nm. Chromatography was carried out at a flowrate of 1.0 ml·min^-1 at room temperature (20 ℃). RESULTS: A better linearity was obtained from 0.256 5 μg to 10.78 μg of nimesulide per ml for plasma with a good correlation coefficient (r=0.999 58,n=8). The mininum detectable concentration was 0.2 μg·ml^-1 for plasma. CONCLUSION: Drug concentration in plasma was determined by HPLC method following a single oral dose of 100 mg was given to 8 adult male volunteers. The procedure is presently being used in case work.