The disposition of sulindac, a new nonsteroid anti-inflammatory drug, has been studied in normal volunteers in five separate clinical studies. Based upon material balance considerations, a minimum of approximately 88% of an oral dose is absorbed. The major biotransformations involve irreversible oxidation of the sulfinyl group of sulindac to sulfone and reduction to the corresponding sulfide. The latter, which all available evidence indicates to be the pharmacologically active form of sulindac, is not excreted in urine, and has an apparent terminal half-life of 18.2 hr, well suited to twice daily dosage of its pro-drug. Following twice daily dosage of sulindac for 5 days, plasma levels of sulfide approach an apparent steady state with concentrations varying only within a twofold range over each dosage interval. The reversible biotransformation between sulindac and its active sulfide metabolite provides the basis for two therapeutic advantages relating to the gastrointestinal intolerance usually associated with anti-inflammatory drugs: (1) circumvention of initial exposure of gastric and small intestinal mucosa to the active form of the drug and (2) maintenance of systemic levels of active drug by means of enterohepatic recycling, principally of inactive pro-drug.