The effects of treadmill exercise and swimming on cell proliferation in the dentate gyrus and the involvement of opioid receptors were investigated via 5-bromo-2'-deoxyuridine (BrdU) immunohistochemistry. Sprague-Dawley rats of the running groups were made to run on treadmill for 30 min each day, while those of the swimming groups were made to swim for 5 min each day over 3 consecutive days. In addition, intensity-dependency of the effects of running and swimming on cell proliferation was investigated. The present results demonstrate that both treadmill running and swimming increase the number of BrdU-labeled cells in the dentate gyrus and these increasing effects are more potent in light intensity and 5 min swimming groups. Treatment with naloxone increases cell proliferation in the dentate gyrus in all groups, although this increase is statistically insignificant.