A rapid, specific and sensitive ultra-performance liquid chromatography/tandem mass spectrometry (UPLC/MS/MS) method has been established for simultaneous quantitation of psoralen and isopsoralen in rat plasma. Plasma samples were pretreated by direct protein precipitation with acetonitrile. Chromatographic separations were performed on an ACQUITY UPLC BEH C(18) column (50mmx2.1mm, i.d., 1.7microm) at 35 degrees C with a linear gradient of acetonitrile and 0.1% formic acid in water at a flow rate of 0.3mL/min. The two isomers were satisfactorily separated (R=1.7) with a runtime of 4min. Psoralen, isopsoralen, and the internal standard (IS) furazolidone were ionized with an APCI source operated in positive ion mode. The MS/MS transitions used for monitoring were at m/z 187.0-->130.9 for psoralen and isopsoralen, and m/z 225.9-->121.9 for IS. Calibration curve was linear over the concentration range of 1-500ng/mL with the lower limit of quantitation of 1ng/mL for both isomers. The mean extraction recoveries were 78.5+/-6.7% and 81.9+/-8.0% for psoralen and isopsoralen, respectively. The intra- and inter-day precisions were less than 5.6% and 5.2%, and the accuracy was within +/-2.1% for both isomers. No matrix effect was observed in this method. Psoralen and isopsoralen were stable during all storage, pretreatment and analytical periods. The validated method has been successfully applied to a pharmacokinetic study of psoralen and isopsoralen after oral administration of Haigou Pill to rats.