Fifty female CBA mice were given melatonin with drinking water (20 mg/l) for 5 consecutive days monthly, beginning from the age of 6 months, until natural death. Another 50 intact mice were used as controls. Melatonin failed to significantly influence body weight or food consumption. Age-related switching-off of estrus function was delayed, body temperature decreased. Somewhat decreased motor activity did not affect physical one or endurance. Increase in life span led to higher spontaneous tumor incidence. Another experiment using 20 animals of the same line showed melatonin to inhibit free-radical processes. A conclusion was drawn that caution should be exercised before melatonin is recommended for long-term administration as a geroprotector.