A simple high performance liquid chromatography (HPLC) method was developed for the determination of butein in rat serum. The method involved the deproteinization followed by injection into a Luna C8 column. Butein was eluted at 3.8 min at a flow rate of 0.2 ml/min with the mobile phase of acetonitrile-ammonium formate (10mM, pH 3.0) (35:65, v/v). The standard curve was linear (r(2)=0.995) over the concentration range of 0.1-10 microg/ml. The coefficient of variation (CV) of intra- and inter-assay ranged from 2.7 to 7.5% and 6.0 to 7.5%, respectively. The limit of quantification was 0.1 microg/ml using a serum sample of 50 microl. This method was applied to a pharmacokinetic study after intravenous injection of butein (5mg/kg) to rats.