The mode of action of Sulfasalazine is still under investigation, but may be related to the anti inflammatory and/or immunomodulatory properties that have been observed in animal and in vitro models, to its affinity for connective tissue, and/or to the relatively high concentration it reaches in serous fluids, the liver and intestinal walls, as demonstrated in autoradiographic studies in animals. In ulcerative colitis, clinical studies utilizing rectal administration of Sulfasalazine, Sulfapyridine and 5-Aminosalyclic Acid have indicated that the major therapeutic action may reside in the 5-Aminosalyclic Acid moiety. The relative contribution of the parent drug and the major metabolites in rheumatoid arthritis is unknown.