Sambucus chinensis L. is a native perennial herb distributed throughout China. In traditional Chinese medicine (TCM), this herb is known as Lu-Ying. Ursolic acid is the major effective constituent of Lu-Ying. A rapid, sensitive, and accurate liquid chromatography-mass spectrometry (LC-MS) method for the determination of ursolic acid in rat plasma was developed and validated. Plasma samples taken from rats that had received Lu-Ying extract orally were acidified with acetic acid and then extracted with a mixture of hexane-dichloromethane-2-propanol (20:10:1,v/v/v). Separation of ursolic acid was accomplished on a C18 column interfaced with a single quadrupole mass spectrometer. The mobile phase consisting of methanol and water (95:5, v/v) was delivered at a flow rate of 1.0 ml/min. Atmospheric pressure chemical ionization was operated in negative-ion mode. Using selected ion-monitoring mode, the deprotonat-ed molecules [M-H] at m/z 455 and 469 were used to quantify ursolic acid and glycyrrhetic acid (internal standard), respectively. The assay was shown to be linear over the range of 10-1000 ng/ml (r> 0.9960) with a lower limit of quantification of 10 ng/ml. The method was shown to be reproducible and reliable with intraday precision below 7.8%, inter-day precision below 8.1%, accuracy within ±4.3%, and mean extraction recovery excess of 83.6%, which were all calculated from the blank plasma sample spiked with ursolic acid at three concentrations of 20, 200, and 800ng/ml. The LC-MS method has been successfully applied to pharmacokinetic studies of ursolic acid after oral administration of LuYing ethanolic extract (at a dose containing 80.32 mg/kg ursolic acid) to rats. The main pharmacokinetic parameters were: ℓ1/2, 4.3 h; Ke,0.16 1/h; ℓmax, 1.0 h; Cmax, 294.8 ng/ml; A UC0-t and A UC0- 1007.1 ng•h/ml and 1175.3 ng•h/ ml, respectively.