5-Aminosalicylic acid presently is believed to represent the therapeutically active moiety of the sulfasalazine molecule in the treatment of inflammatory bowel disease. The metabolism of this compound, however, has not been studied in detail. In this paper we provide evidence that 5-aminosalicylic acid is acetylated to N-acetyl-aminosalicylic acid in homogenates from colonic biopsy specimens (370 +/- 20 nmol/g wet wt or 2.9 +/- 0.9 nmol/mg.min, n = 10), whereas acetylation in fecal samples was only small (13.0 +/- 3.0 nmol/g). Mucosal N-acetylation was rapid, cofactor- and pH-dependent, and could be enriched in the cytosolic fraction. In contrast, fecal acetylation was slow and did not depend on the presence of acetyl-coenzyme A. There were neither significant differences of acetylation between patients and controls nor a significant correlation to the individual acetylation phenotype. From our results we believe that presystemic acetylation of 5-aminosalicylic acid may be mainly mediated by a colonic mucosal enzyme and only to a small extent by fecal (bacterial) processes.