Enemas containing 1000 mg 5-ASA were administered to patients with active distal colitis in three separate studies: as a single dose in a neutral solution (pH 7.4); as a single dose in a slightly acidic, buffered suspension (pH 4.8); and as multiple doses once a day for 10 days with the acidic enema. 5-ASA was relatively rapidly absorbed from the neutral solution, resulting in plasma concentrations of 5-ASA sometimes two to three times higher than those found after peroral salazosulphapyridine (SASP) treatment. In contrast, absorption from the acidic enema was reduced and/or prolonged, giving plasma concentrations similar to those found during oral SASP treatment. After repeated doses of the acidic enema, plasma concentrations after an enema resembled those seen after the single dose. Urinary excretion was significantly lower, suggesting a reduced fraction of absorption at steady-state conditions. No side effects were observed, and no local irritation was reported. An acidic buffer suspension with 5-ASA seems to be safe for use as enema and deserves further clinical testing for treatment of distal ulcerative colitis.