In 24 subjects without upper gastrointestinal lesions, gastric pH was measured from 30 min before until 90 min after the administration of diclofenac sodium (50 mg), piroxicam (20 mg), or 500 mg acetylsalicylic acid. In all these cases a drop of gastric pH was recorded, which started with all the drugs 15 min after their administration and lasted throughout the recording. Pre-treatment with rosaprostol (2 g given 30 min before the start of the trial) prevented the drop in pH. Twenty subjects with chronic joint diseases were divided into two groups in a cross-over double-blind randomized experimental design. One group received piroxicam (20 mg) + rosaprostol (2 g) daily; the other group was treated with piroxicam 20 mg + placebo. The patients were clinically reviewed every week in a month and questioned about their symptoms. Statistical analysis demonstrated that patients with articular diseases treated with NSAIDs + rosaprostol exhibited a frequency and severity of symptoms lower than those recorded in subjects receiving NSAIDs + placebo. Rosaprostol was found to be capable of antagonizing the variations of gastric acid output induced by the oral administration of NSAIDs, and to prevent and treat the occurrence of digestive disorders when given in combination with NSAIDs. These effects result from the action of rosaprostol on the mucosal barrier, and this cytoprotective action is confirmed by the present study with continuous measurements of gastric pH.