The impact of age on the effect of treadmill exercise on cell proliferation in the dentate gyrus of rats was investigated via 5-bromo-2'-deoxyuridine immunohistochemistry. Animals of different ages were used: 4-week-old, 8-week-old, and 62-week-old. Based upon the present study, the most prominent cell proliferation in the dentate gyrus was observed in the 4-week-old rats, and decreased in direct relation to the age of the animals. In addition, although treadmill exercise increased cell proliferation in the dentate gyrus of animals in all age groups, the most potent enhancing effect appeared in the 8-week-old rats. The present results demonstrate that age is an important factor in the regulation of cell proliferation in the dentate gyrus and that the enhancing effect of the treadmill exercise on cell proliferation also depends on age status.