Japanese monkeys were studied for one month from 6 months (short-term) after inoculation of Helicobacter pylori, and two years (long-term). In the short-term study, macroscopic and histological gastritis were observed for 5 infected monkeys. The gastritis score, the ammonia concentration in the gastric secretion and the level of serum antibody (IgG) were higher for the infected group than for the control. In the long term study, H. pylori was always recovered for 2 years in the infected group, and the gastritis score was higher than the control. The intracellular periodic acid-Schiff (PAS) positive substances in the infected group decreased than that of the control. The tall of pyloric gland in the infected group was significantly decreased on 6 months from 1.5 years. These results suggest that the Japanese monkey can be used as experimental model of H. pylori infection and that H. pylori can induce chronic active gastritis similar to human.