Abstract: ABSTRACT: OBJECTIVE To establish a rapid and sensitive method for quantification of dexamethasone in equine plasma using liquid chromatography-tandem mass spectrometry (LC-MS/MS). METHODS Analyte was recovered from plasma by liquid-liquid extraction using methyl tert-butyl ether (MTBE), separated on a C18 column, and analyzed in positive electrospray ionization mode. Multiple-reaction monitoring (MRM) was employed for quantification. RESULTS The linear dynamic range of the method was 0.01-10 ng·mL-1. The precision and accuracy at 0.05, 0.5 and 5 ng·mL?1 of dexamethasone during intra-day assay were 1.6-8.9% and 98-120%, respectively. For inter-day assay, precision and accuracy at the same three concentrations were 2.2-9.7% and 104-120%, respectively. CONCLUSION The present method is fast, simple, sensitive and reproducibly reliable, could be applied to the pharmacokinetics study of dexamethasone.