AbstractThe free diphenol oxyphenisatin (I) and its sulphuric acid (II) and acetic acid(III)di‐esters are compared.Idisappered rapidly from the lumen of tied jejunal loopsin vivo; its disapperance was associated with an inhibition of normal fluid absorption and an increase in loop tissue wet weight.IIdisappeared slowly, and did neither inhibit fluid absorption nor cause an increase in wet weight. Large fractions ofIconjugates were excreted in the bile 0‐6, 6‐12 and 12‐18 hrs following intragastric administration ofI; afterIIandIIImuch smaller fractions were excreted, and their time pattern differed from that ofI. The absorption and biliary excretion experiments when combined indicate that following intragastric administration in the intact rat, the bulk ofIwill undergo enterohepatic circulation during its small intestinal transit, whileIIandIIIpass largely unabsorbed to the colon. In spite of their direct transit, however, the laxative action of bothIIandIIIin concious rats was slower in onset than ofI. In this respect,IIdiffered strikingly from the sulphuric acid di‐ester of desacetyl‐bisacodyl (IV), which acted even more promptly thanI. As in the jejunum,IIwas inactiveper sein tied colonic loops; liberation of freeIbeing required to elicite inhibition of colonic electrolyte and fluid absorption. The reason for the long laxative latency period ofIIandIIIappears then mainly to be slow colonic hydrolysis of these prodrugs. Supplementary experiments supported this view as far as the water solubleIIis concerned, and slow hydrolysis is very likely for such a sparingly soluble