Fenbufen (Cinopal) is an orally active nonsteroidal anti-inflammatory drug with analgesic and antipyretic activity. Like clinically used nonsteroidal anti-inflammatory drugs, it shows activity in a wide spectrum of laboratory tests in mice, rats, guinea pigs, and dogs. Fenbufen has a long duration of anti-inflammatory and analgesic activity. Mechanistic studies indicate that fenbufen has no intrinsic effect on cyclooxygenase activity, whereas its major metabolite, biphenylacetic acid, is a potent inhibitor of prostaglandin synthesis. These observations indicate that fenbufen is a pro-drug and account for its low ulcerogenic potential. Anti-inflammatory pro-drugs that are readily metabolized to the biologically active molecule are expected to retain a favorable anti-inflammatory to ulcerogenic ratio because the gastrointestinal tract is not exposed to a large concentration of the active molecule. Comparative studies in the type II collagen arthritis model indicate that the anti-inflammatory properties of fenbufen are more potent than those of a second nonsteroidal anti-inflammatory drug, sulindac.