A sensitive and specific high-performance liquid chromatographic method was developed for the determination of ketoprofen [2-(3-benzoylphenyl)propionic acid] in plasma and urine. The method includes an extraction of the drug and the internal standard [2-(4-benzoylphenyl)butyric acid] into ether from acidified plasma. The organic phase is evaporated, and the residue is dissolved in the mobile phase (acetonitrile-0.02 M phosphate buffer, pH 3) (45:55). A 20-microliter aliquot is analyzed on a reversed-phase column. The accuracy is within 1.5% for therapeutic concentrations, and the coefficients of variation are 5.5 and 3.4% for 2 and 10 micrograms/ml, respectively. For the urine assay, the accuracy is within 3%, and the cofficients of variation are 1.9 and 1.7% for 3 and 50 micrograms/ml, respectively. This method was applied successfully to the determination of ketoprofen in humans for pharmacokinetic studies.