Effects of milk, prostaglandin, and antacid on experimentally induced duodenitis in the rat. Use of myeloperoxidase as an index of inflammation. Ulcerogenesis of the duodenal mucosa frequently involves an inflammatory reaction with infiltration of leukocytes. Measurement of neutrophil myeloperoxidase activity might thus be a sensitive indicator of damage, before visible lesions occur. To test this possibility, a rat model for duodenal injury was used where fasted animals were treated with indomethacin and histamine-diHCl. Twenty-four hours after indomethacin treatment, duodenal tissues were collected for histochemical staining and biochemical assay for myeloperoxidase activity. Indomethacin- and histamine-challenged rats had significantly elevated myeloperoxidase activity compared to unchallenged controls (P less than 0.05) for both histochemistry and biochemistry. There was also a significant correlation between these two parameters (r = 0.68, P less than 0.001). The duodenal injury model then was used to test the effectiveness of known gastric protective agents. Results indicated that milk and buttermilk did not aggravate or protect against duodenal injury, while antacid and prostaglandin did significantly protect against inflammation (P less than 0.02). We concluded that measurement of myeloperoxidase activity is a sensitive and potentially useful estimate of duodenal injury that can be valuable in assessing ulcerogenesis and healing. 