Abstract: A highly sensitive liquid chromatography-tandem mass spectrometric(LC-MS/MS) method was developed for the determination of actinoside E in rat plasma.The analytes were extracted by ethyl acetate and an analogue of actinoside F was used as the internal standard.The mobile phase consisted of methanol-water(50:50,V/V) containing 0.1% formic acid was delivered at a flow rate of 0.3 mLmin-1 to a Zorbax SB-C18 column(100 mm2.1 mm,3.5 m).The detection was performed by electrospray ionization mass spectrometry in the negative multiple reaction monitoring mode with a chromatograph run time of 3.0 min.Calibration curves of actinoside E were linear in the range of 0.5-2 500 ngmL-1.In this range,intra-and inter-day precision ranged from 1.7% to 7.5% and 2.0% to 8.9%,respectively.The accuracy ranged from 95.7% to 108.6%,and extraction recovery from 83.2% to 85.5%.This method was successfully applied to a pharmacokinetic study of actinoside E in rats after intravenous(5 mgkg-1) and oral(100 mgkg-1) administration,and the results showed that actinoside E was poorly absorbed with an absolute bioavailability being approximately 0.27%.