Caesalpinia sappan L. is a traditional medicinal plant which is used for promoting blood circulation and cerebral apoplexy therapy in China. Previous reports showed that the extracts of Caesalpinia sappan L. could exert vasorelaxant activity and anti-inflammation activity. Protosappanin B is a major constituent of C. sappan L., and showed several important bioactivities. The separation was achieved by an Acquity UPLC BEH Symmetry Shield RP18 column (1.7 mu m, 2.1 x 100 mm) column with the gradient mobile phase consisting of 5mM ammonium acetate aqueous solution and acetonitrile. Detection was carried out by using negative-ion electrospray tandem mass spectrometry via multiple reaction monitoring. Plasma samples were preprocessed by an extraction with ethyl acetate, and apigenin was used as internal standard. The current UPLC-MS/MS assay was validated for linearity, accuracy, intraday and interday precisions, stability, matrix effects and extraction recovery. After oral and intravenous administration, the main pharmacokinetic parameters were as follows: peak concentrations, 83.5 +/- 46.2 and 1329.6 +/- 343.6 ng/mL; areas under the concentration-time curve, 161.9 +/- 69.7 and 264.9 +/- 56.3 mu g h/L; and half-lives, 3.4 +/- 0.9 and 0.3 +/- 0.1 h, respectively. The absolute bioavailability in rats of protosappanin B was 12.2%. The method has been successfully applied to a pharmacokinetic and bioavailability study of protosappanin B in rats.