For the evaluation of gastric irritation of oxaprozin in comparison to indomethacin and acetylsalicylic acid, a study was carried out with eight healthy male volunteers, investigating doses of 600 and 1200 mg of oxaprozin compared to therapeutical equivalents of 50 mg indomethacin and 1000 mg acetylsalicylic acid. Gastric irritation was checked with the model of the transmural gastric potential difference. The model is based on the assumption that a change in electric tension caused by a lesion of the gastric mucosa, which leads to an increasing permeability of the cell membrane for electrolytes, is a sensitive parameter for cell disintegrity. The results of the study show that oxaprozin has less irritative potency than indomethacin and can thus be qualified as an antiinflammatory drug with a minimum of gastric irritation.