The effects of 16,16-dimethyl prostaglandin E2 PGE2 on gastric mucosal blood flow and gastric mucosal damage were tested in rats given indomethacin. Blood flow was measured by hydrogen gas clearance. Indomethacin given intragastrically reduced the blood flow in nonlesion areas and the levels of PGE2 and 6-keto-PGF1 alpha in the gastric mucosa and caused mucosal damage in a dose-related way. Indomethacin (20 mg/kg) significantly reduced the blood flow 30 min after administration, and the reduction increased until 90 min. Then the flow plateaued until 240 min. Gastric mucosal damage caused by 20 mg/kg of indomethacin and evaluated by only gross observations, began at 60 min and developed with time until 240 min after administration. 16,16-Dimethyl PGE2 (5 micrograms/kg) did not affect the reduction of blood flow caused by indomethacin during 240 min of measurements, but it significantly inhibited the indomethacin-induced mucosal damage evaluated by gross observations. These results suggest that prevention by 16,16-dimethyl PGE2 of grossly observed gastric mucosal damage caused by indomethacin was not related by preservation of the gastric mucosal blood flow in the areas without lesions.