The rapid,sensitive,and selective liquid chromatography–electrospray ionization–tandem mass spectrometry method (LC–ESI-MS/MS) for the simultaneous estimation and pharmacokinetic investigation of glimepiride and pioglitazone in human plasma has been developed and fully validated.Glimepiride and pioglitazone,compounds which exert synergistic effects on blood glucose control,were investigated in human plasma using deuterium-labeled analogues as internal standards (IS).Liquid-liquid extraction was carried out on 0.2 mL of human plasma using ethyl acetate,and chromatographic separation was performed on an Agilent Eclipse plus C18 column (4.6 mm × 100 mm,3.5 μm) using a mobile phase consisting of methanol-water-formic acid (95:5:0.1,v/v/v,plus 5 mM ammonium acetate) at a flow rate of 0.8 mL/min.To quantify glimepiride,pioglitazone and their IS,multiple reaction monitoring (MRM) transitions of m/z 491.2→352.2,m/z 496.2→357.2,m/z 357.2→134.2 and m/z 361.2→138.2 were performed in positive mode.The total run time was 3.0 min and the elution time was about 2.4 min.The method exhibited good separation of analytes,without interference from endogenous substances.The linear calibration curves were 0.2-250 ng/mL for glimepiride and 0.2-1250 ng/mL for pioglitazone; the lower limit of quantification (LLOQ) was 0.2 ng/mL for both analytes.Intra- and inter-day reproducibility was less than 10% for glimepiride and less than 5% for pioglitazone,with relative errors ranging from -8.00% to 2.80% at the three concentrations of analytes used for quality control (QC).The matrix effect was negligible and recoveries were similar for each analyte and its IS.Glimepiride and pioglitazone were found to be stable under the assay conditions and the method was successfully applied to the evaluation of pharmacokinetic studies of glimepiride and pioglitazone,following oral doses of 2 mg glimepiride tablets and 15 mg pioglitazone tablets to 16 healthy volunteers.