Dextran-5-aminosalicylic acid ester (dextran-5-ASA) was synthesized as a colon-specific prodrug of 5-aminosalicylic acid (5-ASA) which is active against inflammatory bowel diseases. Chemical stability of dextran-5-ASA in the bath of pH 1.2 or 6.8 was investigated at 37°C for 6 hrs, and 5-ASA was not released on such conditions. Depolymerization (%) of dextran-5-ASA by dextranase with the degree of substitution (DS) of 18, 23, or 30 was 92, 62 or 45 in 8 hrs respectively, but was not affected by the MW of dextran (9,000, 40,600, 80,200 or 580,000). Distribution of 5-ASA in dextran, determined by gel filtration chromatography, appeared to be relatively uniform. Incubation of dextran-5-ASA (DS 18) in cecal contents of rats released 20% (28 g) and 35% (49 g) of 5-ASA in 8 hrs and 24 hrs, respectively, but no 5-ASA was liberated from small intestinal contents.