The influence of intestinal transit time on the release of 5-aminosalicylic acid (5-ASA) from a peroral, slow-release preparation (Pentasa) was studied at steady state in seven healthy volunteers. Daily dose was 1500 mg Pentasa, normal transit time (NTT) was 24 h (16-26 h) and accelerated transit time (ATT), caused by a laxative, was 5 h (4-9 h). Median total recovery (24 h, 5-ASA + acetyl-5-ASA) was 87% (61-129%) (NTT) and 81% (56-100%) (ATT), respectively, (P greater than 0.10). The total faecal excretion of 5-ASA (per cent of dose) increased from 16%, (9-21%) (NTT) to 29%, (16-38%) (ATT) (P less than 0.02). Free 5-ASA rose from 12% (4-19%) to 17% (10-25%), the retained part (in granules) from 4% (2-5%) to 12% (4-24%). Urinary excretion decreased correspondingly from 32% (19-59%) to 21% (11-38%), predominantly as Ac-5-ASA (P less than 0.05). Mean plasma Ac-5-ASA concentration decreased from 1.42 micrograms ml-1 to 0.86 microgram ml-1 (P less than 0.05). An almost complete release of 5-ASA from Pentasa takes place during NTT. At ATT conditions about 88% is released, indicating Pentasa to be an acceptable source of 5-ASA in diarrhoeal states.