The effect of the diphenolic laxatives bisacodyl and deacetylbisacodyl on mucus secretion and fluid, sodium and potassium net transport was studied in rat colon perfused in vivo. Mucus output in the effluent was determined as total protein-bound hexose. Deacetylbisacodyl was more potent than the parent compound and was used to investigate dose-response relationships. At a low concentration (0.1 mg/dl), mucus and potassium secretion were stimulated whereas sodium and fluid absorption were inhibited, or converted to secretion, only at higher concentrations (0.5-3.0 mg/dl). All effects were dose-dependent and reversible within 1 h. With longer lasting perfusion of deacetylbisacodyl, mucus appeared in two peaks, one initial peak and another after 4 h. The late peak contained newly synthetized glycoproteins as indicated by the incorporation of intravenously injected [14C]galactose. It is concluded that stimulation of mucus secretion and synthesis contributes to the laxative action of bisacodyl. The effects of low versus high concentrations suggests that part of the potassium secretion is due to mucus release.