Abstract: MTP-131 is a tetrapeptide under development for treating diabetes. MTP-131 contains a lysine, an arginine, a phenylalanine and a modified tyrosine in its structure. The quantification of MTP-131 in rat plasma was developed and validated by liquid chromatography-tandem mass spectrometry(LC-MS/MS). MTP-131 was separated from plasma by Waters Oasis^® HLB cartridge solid phase extraction, then separated by C₁₈ column. The mobile phase consisted of V(acetonitrile):V(10 mmol L^－1 ammonium acetate):V(acetic acid)＝30:70:0.04. A mass spectrometer equipped with electrospray ionization source was used as a detector and operated in the positive ion mode. Selected reaction monitoring(SRM) using the double protonated molecules of M＋2H^2＋ for MTP-131, three main fragment ions of m/z 70, 84, 20 were used for the quantification. The d5-MTP-131 was used as the internal standard. The method exhibites a linear range of 1.0-10 000μg L^－1 for MTP-131 with a lower limit of quantification at 1.0 μg L^－1. The intra- and inter-assay precisions are below 9.9%, and relative errors within 6.0%. The mean extraction recovery of MTP-131 is above 80%. Under the selected chromatographic conditions and sample preparation conditions, there is no significant matrix effect of MTP-131 at low or high concentrations. The validated method proves to be sensitive, rapid, and is applied successfully for the pharmacokinetic study of MTP-131 in rat.