The human pharmacokinetics of 5-aminosalicylic acid (5-ASA), the active moiety of salazosulphapyridine (SASP), is only known from studies in which rapid absorption has been deliberately avoided. The present investigation demonstrates that pure 5-ASA is absorbed extremely quickly when given as an instillation in the proximal part of the small bowel, and acetylation follows immediately. The metabolite is excreted very rapidly by the liver in small amounts, while the major part is eliminated renally.