OBJECTIVE： To establish a method for the determination of mesalazine concentration in plasma of Beagle dogs， and to study the pharmacokinetics of Mesalazine sustained-release tablets in Beagle dogs. METHODS： HPLC-MS/MS method was used to determine the plasma concentration of mesalazine. Using salicylic acid as internal standard， after pre-treatment of acetonitrile precipitated protein， the determination was performed on Inertsil ODS-3 column with mobile phase consisted of acetonitrile-10 mmol/L ammonium acetate （containing 0.2％ formic acid） solution （85 ∶ 15，V/V）. ESI source （negative ion mode） detection was adopted in MRM mode， and the ion pair for quantity analysis were m/z 152.0→107.9 （mesalazine） and m/z 137.0→92.9 （internal standard）. Six Beagle dogs were selected and given single dose （1.2 g） and multiple dose （1.2 g each day， for consecutive 7 d） of Mesalazine sustained-release tablets. Blood sample was collected at different time points， and plasma concentration was determined. DAS 3.3.1 software was used to calculate pharmacokinetic parameters of different doses administration and accumulation factor （R） of multiple dose administration. Pharmacokinetic parameters were compared by using SPSS 16.0 software. RESULTS： The linear range of mesalazine concentration were 0.1-100 μg/mL （r＝0.999 1）， and minimum quantification limit was 0.1 μg/mL. The RSDs of inter-batch and intra-batch precision and matrix effect were all less than 11％. Relative error （RE） of accuracy ranged －0.75％-10.45％； extraction recovery was higher than 60％ （RSD＜5％）. RSDs of stability tests were lower than 10％， and RE was within ±15％. After administration of single dose and multiple dose， there was no statistical significance in peak concentration （cmax）， peak time （tmax）， area under curve of plasma concentration-time curve （AUC0-t， AUC0-∞）， retention time （MRT0-t）， retention variance （VRT0-t）， clearance rate （CLz） or elimination half life （t1/2z） of mesalazine （P＞0.05）. The R of multiple dose was less than 1.01. CONCLUSIONS： The method is accurate， rapid， sensitive and specific， which is suitable for pharmacokinetic study of Mesalazine sustained-release tablets in Beagle dogs. After single dose and multiple dose administration，mesalazine has same pharmacokinetic characteristics in Beagle dogs， but no drug accumulation is found.