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A simple method for determination of mosapride citrate and its metabolite, des-p-fluorobenzyl mosapride (M-1), in equine muscle, liver, kidney, adipose tissue and intestine by liquid chromatography-tandem mass spectrometry has been developed. (+/-)-4-Amino-5-chloro-2-ethoxy-N-[[4-(2-chlorobenzyl)morpholinyl]methyl]benzamide was used as an internal standard. The analytes and internal standard were spiked and extracted from tissues by acetonitrile. The chromatographic separation was performed on a reversed-phase TSK-GEL SUPER ODS column with a mobile phase of acetonitrile-0.05% (v/v) formic acid containing 5 mmol/L nonafluoropentanoic acid (2:3, v/v). The method exhibited a large linear range from 0.0005 to 0.2 microg/mL for both mosapride citrate and M-1 (r>0.9976). In the intra-day assay (n=5), the relative standard deviations (RSDs) ranged from 1.1 to 7.8% for mosapride citrate and 1.6 to 7.2% for M-1. In the inter-day assay (n=3), the RSDs ranged from 1.0 to 13% for mosapride citrate and 0.8 to 11% for M-1. The extraction recovery at 1.28 microg/g of mosapride citrate from equine tissues ranged from 97 to 107%. The lower limit of quantification for mosapride citrate was found to be 0.004 microg/g. Stability studies were carried out at different storage conditions. The method reported is reliable, precise, and accurate and it has the capacity to be used for determination of mosapride citrate and its metabolite in tissue samples.