We subcutaneously injected indomethacin (IDM) to rats and evaluated its effects on the unstirred water layer (UWL) of the small intestine. One hour after IDM administration at a dose of 4, 10, 20 and 50 mg/kg, the thickness of UWL decreased dose-dependently. Serial measurement of UWL 1, 3, 6, 12 and 24 hours after the administration showed early thinning of UWL after 1-3 hours. Many IDM-induced ulcers were shown 24 hours after the injection of IDM at a dose of 10 mg/kg. However, in the group given food during the latter 3 hours between 12 and 15 hours after the drug administration, the number of ulcers was significantly decreased compared with those in the group given food ad libitum or in the group given food during the early 3 hours. When pectin, a soluble dietary fiber, was given together with solid stock food, ulcer formation was significantly inhibited. Synthesis of mucus glycoprotein by tissue organ culture was also decreased in the jejunum in the IDM-treated group. These results suggest that the formation of IDM-induced intestinal ulcer is related to the thing of intestinal UWL and the decrease of the synthesis of intestinal mucus glycoprotein.