Although several recent studies have reported that curing Helicobacter pylori (H. pylori) may result in the development of reflux esophagitis (RE), the mechanisms leading to this complication are unknown. One by product of H. pylori infection is ammonia, which serves as an acid neutralizer. The aim of this study was to clarify whether ammonia, which is produced during H. pylori infection, has a protective effect on the esophagus. Eight-week-old male Sprague-Dawley rats were fasted for 24 hrs. Under anesthesia, both the pylorus and limiting ridge were simultaneously ligated. One hour postligation, 0.3 ml of saline or ammonia at various concentrations was administered intragastrically by gastric intubation. Three hours after ligation, the animals were killed, the esophagus and stomach were removed, and the length of esophageal hemorrhagic erosions was measured. The incidence of RE was 100% (7/7) in the control group, 71% (5/7) in the low-ammonia group, 29% (2/7) in the middle-ammonia group, and 14% (1/7) in the high-ammonia group. The severity of lesions decreased in correspondence to increases in ammonia concentration. The development of RE was significantly inhibited by ammonia in a dose-dependent manner. This study indicates that ammonia protects against development of RE. A decreased amount of ammonia in the stomach might be related to the development of RE after H. pylori eradication therapy.