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A sensitive and specific high-performance liquid chromatography combined with electrospray ionization (ESI) tandemmass spectrometry (LC-MS/MS) method, operating in the positive ionization mode, for quantifying of amlodipine in humanplasma using tizanidine as internal standard (I.S.) was developed and validated. The analyte and I.S. were extracted bysimple one step liquid/liquid extraction with a mixture of diethylether/dichloromethane (70/30, v/v). The chromatographicseparation was performed on a C18 analytical column under isocratic conditions using a mixture of 10mM ammoniumformate/methanol/acetonitrile (30/50/20, v/v/v) as mobile phase at a flow rate of 1.0 mL/min. Total chromatographic runtime was 5.0 min. Detection was performed on a API 2000 QTRAP quadrupole linear ion trap mass spectrometer via turboion spray ionization. Quantitation was performed using multiple reaction monitoring (MRM) mode to study parent ®product ion transitions of m/z 409.4 ® 238.1 for amlodipine and m/z 254.2 ® 44.1 for I.S., respectively. The validation andstability studies were performed according to the Thai FDA guidance for assessment of bioequivalence study in Thailand.The results were within the accepted criteria as stated in the aforementioned guidance. Linearity in plasma was obtained overthe concentration range 0.3-15.0 ng/mL, with a coefficient of determination (r2) of 0.9993. Lower limit of quantification(LLOQ) was identifiable and reproducible at 0.3 ng/mL. The within- and between-run precision values were below 10%and the accuracy was ranged from 94.87 to 102.44% at all three quality controls samples levels. The analyte was found tobe stable in plasma samples under three freeze-thaw cycles, long-term storage (3 months at -20oC), short-term storage (4hours at room temperature), post-preparative and stock-solution stability. The robust and rapid LC-MS/MS method has beensuccessfully applied for routine assay to support bioequivalence or pharmacokinetics studies of amlodipine administeredas a single oral dose (10 mg tablet) to Thai healthy volunteers.