The biliary secretion of sulindac and metabolites after a single 400 mg oral dose of the drug was studied in 3 elective gallbladder surgical patients following placement of an occludable T-tube in the common bile duct. Bile and systemic plasma were sampled at frequent intervals for up to 36 h postdose. The apparent biliary clearance (Vcl,bile) of the prodrug sulindac is about 25 times greater than that of the pharmacologically active sulfide metabolite. The total biliary flux of drug in normal man with an uninterrupted enterohepatic cycle, calculated from Vcl,bile and historic mean plasma drug AUCinfinity0 values, averages 144 and 12.2 per cent of the dose as sulindac and the sulfide metabolite, respectively. Thus, enterohepatic recycling of the drug in man is principally in the form of the prodrug which not only limits exposure of the intestine to the active moiety but also sustains systemic concentrations of active drug upon reabsorption of the prodrug.