AbstractHeterophyllin B (HB) is a cyclic octapeptide isolated from Pseudostellaria heterophylla. HB is used as the quality control index for evaluating P. heterophylla in the Chinese Pharmacopoeia. A rapid and sensitive LC‐ESI‐MS/MS method was developed and validated for the analysis of HB in rat plasma. Sample preparation consisted of a solid‐phase extraction step for the removal of interference and preconcentration of the target analyte HB and the internal standard N‐acetylcysteine before chromatographic analysis by MS/MS detection. The separation of HB and N‐acetylcysteine was performed using a Hypersil GOLDTM C18 column and a mixture of methanol–water (60:40, v/v) containing 10 mmol/L ammonium formate and 0.1% formic acid as the mobile phase. The determination step was optimized in the selected reaction monitoring mode for the highly selective and sensitive quantitation of HB in rat plasma. Intra‐ and inter‐assay precision (as relative standard deviation) was ≤9.1%, and accuracy was between 92.6 and 102.7%. The validated method was successfully applied to quantify HB concentrations up to 7 h after tail intravenous injections of 2.08, 4.16 and 8.32 mg/kg HB in rats. The LC‐MS/MS method identified the relevant pharmacokinetic parameters of HB and its studied analog. Copyright © 2015 John Wiley & Sons, Ltd.