The stability of 5-aminosalicyclic acid and its metabolites has been investigated when stored frozen. N-β- d-Glucopyranosyl-5-aminosalicylic acid was formed in considerable amounts concomitant with a decrease in 5-aminosalicylic acid in plasma samples spiked with 5-aminosalicylic acid as well as in standard solutions of 5-aminosalicylic acid buffered with potassium phosphate between pH 5.5 and pH 8.0 with 4.0 m M glucose added and stored at −20°C. Thus N-β- d-glucopyranosyl-5-aminosalicylic acid migth not, as previously described, be a metabolite of 5-aminosalicylic acid but an artifact formed during storage of plasma samples. The N-glucoside formed could be quantitatively degraded to 5-aminosalicylic acid and glucose by adding 0.2 M potassium phosphate buffer pH 3.0 to the sample prior to the analysis. The metabolites of 5-aminosalicylic acid (N-formyl-5-aminosalicylic acid, N-acetyl-5-aminosalicylic acid and N-butyryl-5-aminosalicylic acid) were found to be stable in plasma stored at −20°C for at least eight months.