A simple and specific high-performance liquid chromatographic (HPLC) method with ultraviolet (UV) absorbance detection has been developed for the determination of isoferulic acid in rat plasma. The plasma samples were deproteinized with methanol after the addition of internal standard (IS) tinidazole. The analysis was performed on a Kromasil C18 column (250 mm x 4.6 mm i.d., 5 microm particle size) with acetonitrile-0.05% phosphoric acid (25:75, v/v) as mobile phase. The linear range was 0.0206-5.15 microg ml(-1) and the lower limit of quantification (LLOQ) was 0.0206 microg ml(-1). The intra- and inter-day relative standard deviations (R.S.D.s%) were less than 11.4 and 12.3%, respectively, and accuracy as relative error (R.E.%) between -6.7 and -1.1%. Mean extraction recovery was above 80%. The validated method was successfully applied to the pharmacokinetic study of isoferulic acid in rat plasma after oral administration of isoferulic acid and Rhizoma Cimicifugae extract.