As a prerequisite to the determination of pharmacokinetic parameters of icariin in rats, an HPLC method using UV detection was developed and validated. Icariin and the internal standard, quercetin, were extracted from plasma samples using ethyl acetate after acidification with 0.05 mol/L NaH2PO4 solution (pH 5.0). Chromatogra- phic separation was achieved on an Agilent XDB C-18 column (250 x 4.6 mm id, 5 mu m) equipped with a Shim-pack GVP-ODS C-18 guard column (10 x 4.6 mm id, 5 mu m) using a mobile phase of ACN/water/acetic acid (31:69:0.4 v/v/v) at a flow rate of 1.0 mL/ min. Detection was at 277 nm. The calibration curve was linear from 0.05 to 100.0 mu g/mL with 0.05 mu g/mL as the lower LOQ (LLOQ) in plasma. The intra- and interday precisions in terms of RSD were lower than 5.7 and 7.8% in rat plasma, respectively. The accuracy in terms of relative error (RE) ranged from - 1.6 to 3.2%. The extraction recoveries of icariin and quercetin were 87.6 and 80.1%, respectively. The main pharmacokinetic parameters for rats were determined after a single intravenous administration of 10 mg/kg icariin: t(1/2), 0.562 +/- 0.200 h; AUC(0-infinity), 8.73 +/- 2.23 mu g center dot h/mL; CLTOT, 20.10 +/- 5.80 L/kg center dot h; V-z, 1.037 +/- 0.631 L/kg; MRT0-infinity, 0.134 +/- 0.040 h; and V-ss, 0.170 +/- 0.097 L/kg.