A rapid, sensitive, and specific liquid chromatography-electrospray ionization-tandem mass spectrometry (LC-ESI-MS-MS) method was developed and validated for simultaneous determination of aconitine (AC), mesaconitine (MA), and hypaconitine (HA), the three toxic constituents from Sini decoction (SND) in rat plasma. After the addition of citalopram as the internal standard (IS), plasma samples were basified with 100 microL 10% ammonium hydroxide, and then extracted with 1 mL ethyl acetate. Chromatographic separation was performed on a CN column (250 mm x 4.6 mm, 5 microm) with a mobile phase of methanol/40 mM ammonium acetate/formic acid (950:45:5, v/v/v) at the flow rate of 1.0 mL/min. Analytes were determined in a triple-quadrupole mass spectrometer in the selected reaction-monitoring (SRM) mode using electrospray source with positive mode. The method was validated over the concentration ranges of 0.01-10 ng/mL for AC, MA, and HA. The variation coefficients were always < 15% for both intraday and interday precision for each analyte. Mean accuracies were also within +/-15%. The method was proved to be sensitive, rapid, specific, accurate, and reproducible. It has been successfully applied to the pharmacokinetics study on rats after oral administration of SND.