A rapid, sensitive and selective liquid chromatography-tandem mass spectrometric (LC-MS/MS) method was developed and validated for the determination of Timosaponin B-II (TB-II), a pharmacologically active constituent isolated from Anemarrhena asphodeloides. This method was used to examine the pharmacokinetics and bioavailability of TB-II in rats using ginsenoside Re as an internal standard. After simple protein precipitation of the plasma samples with acetonitrile, the analytes were separated on an ODS column (150 mm x 2.1 mm i.d., 5 microm) with the mobile phase of acetonitrile-water (35:65, v/v) containing 0.05% formic acid and detected by electrospray ionization mass spectrometry in the negative multiple reaction monitoring (MRM) mode with a chromatographic run time of 3.0 min. The calibration curves were linear over the range of 5-15,000 ng/ml and the lower limit of quantification (LLOQ) was 5 ng/ml in rat plasma. In this range, relative standard deviations (R.S.D.) were <7.4% for intra-day precision and <9.0% for inter-day precision. The accuracy was within the range of 97.7-107.3%. The method was successfully applied to assess the pharmacokinetics and oral bioavailability of TB-II after intravenous and oral administration in rats, with the oral bioavailability being only 1.1%.