A high-performance liquid chromatography-electrospray ionization tandem mass spectrometry (HPLC-ESI- MS/MS) method was established for the quantitative and qualitative analysis of potassium dehydroandrographolidi succinas (DAS-K) and its degradation products, respectively, in DAS-K injection, a widely used drug in China. The analytical HPLC was performed on a Waters RP-C18 column using 5 micromol m(-1) ammonium acetate (pH 3.0, adjusted by formic acid) and methanol as the mobile phase. Detection was performed on a Micromass Quattro micro triple quadrupole mass spectrometer with positive electrospray ionization. Three major degradation products were separated, detected and identified simultaneously by HPLC-ESI-MS/MS results. An HPLC method for the assay of DAS-K in its injection was then established. The multiple reaction monitoring function (m/z 550.14-->m/z 297.04 for DAS-K) enabled the quantification of DAS-K down to 0.2 ng m(-1). The calibration graph was linear (r better than 0.999, n = 7) in the concentration range 2.0-100 ng mL(-1). The values for the intra- and inter-day relative standard deviation (RSD) were less than 1.7% and 2.5% for DAS-K. The average recovery rate was over 98%, with an RSD less than 3.0%. This is the first report on the degradation products in DAS-K injection.