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.