The effect of a long-term oral ammonia administration on immunoreactive-somatostatin concentrations was investigated in rat stomach. The gastric ir-somatostatin concentrations in the group treated with 0.01% ammonia (pH 9.6) for four weeks were significantly higher than those in both the group treated with 0.1% ammonia (pH 10.4), 0.1 mM-NaOH (pH 9.6), or distilled water (pH 7.0) for four weeks and the group treated with 0.01% ammonia for two weeks. On the contrary, ir-somatostatin levels in the gastric juice and serum tended to decrease with ammonia administration. Further, ammonia administration significantly induced the decrease in mucosal thickness in the pyloric gland area and parietal cell numbers in a dose- and time-dependent manner. From these findings, it was suggested that a long-term oral treatment with 0.01% ammonia, which was clinically estimated as the concentration of the gastric juice in patients with Helicobacter pylori infection, induced not only atrophic changes on gastric mucosa, but the inhibitory effect on somatostatin secretion in rat stomach.