We previously reported that the gastric mucosa emits fluorescence of porphyrins at the onset of gastric lesions induced by hemorrhagic shock. In this study, we investigated whether the fluorescent substance concerns with the gastric mucosal injuries induced by diflofenac, a nonsteroidal antiinflammatory drug (NSAID). In the gastric mucosa treated with diclofenac, lesions were generated and myeloperoxidase activity increased. Diclofenac administration also increased thiobarbituric acid-reactive substances, a index of tissue peroxidation. After diclofenac treatment, the gastric mucosal fluorescence intensities rose. HPLC analysis demonstrated that the fluorescent substances were mesoporphyrin and protoporphyrin, which were the same as found in hemorrhagic shock. Pretreatment of the tissue with radical scavenging substances, catalase and troxipide, restrained the increase of mucosal fluorescence intensity, tissue peroxidation, and lesion formation. These findings indicate that diclofenac treatment induced the generation of porphyrins as well as tissue peroxidation in gastric mucosal tissue. This study suggests that autofluorescence observation is a useful tool to identify diclofenac-induced gastric injury.