Abstract: A liquid chromatographic-tandem mass spectrometric (LC-MS/MS) method was developed for the determination of budesonide in dog plasma. Budesonide and the internal standard triamcinolone acetonide were separated from plasma by alkalinized liquid-liquid extraction with ethyl acetate. Chromatographic separation was performed on a Capcell Pak C₁₈ MG column with the mobile phase consisted of acetonitrile - 5 mmol·L^-1 ammonium acetate (60∶40, v/v) at a flow-rate of 0.50 mL·min^-1. A tandem mass spectrometer equipped with electrospray ionization source was used as detector and operated in the negative ion mode. Quantification was performed using multiple reaction monitoring (MRM) of the transitions m/z 489 → m/z 357 and m/z 493 → m/z 413 for budesonide and the internal standard, respectively. The linear calibration curves were obtained in the concentration range of 25.0-2 000 pg·mL^-1. The lower limit of quantification was 25.0 pg·mL^-1. The intra- and inter-day relative standard deviation over the entire concentration range was less than 15%. The accuracy was in the range of -8.1% to -1.7% in terms of relative error. The method was applied to a pharmacokinetic study of budesonide controlled-release capsules in Beagle dogs. Maximal budesonide plasma level was observed after (3.5±3.3) h and the C_max was (786±498) pg·mL^-1 after a single oral administration of 9 mg budesonide capsules, C_max was increased to (2 142±1 515) pg·mL^-1 after multiple oral administration (9 mg×5 d) of budesonide capsules. This method was selective and rapid, and the sensitivity was sufficient for the purpose of the pharmacokinetic study of budesonide controlled-release formulation.