Oxidative stress is the major factor affecting different organs during aging. Mitochondria are considered to be a source of endogenous oxidants, concentrations of which can be maintained at a low level by antioxidant enzymes. At present, the aging process is considered to be tightly related with mitochondrial dysfunction, one of the reasons of which might be an increased sensitivity to induction of permeability transition pore in the inner membrane of mitochondria. Currently, the role of melatonin, concentration of which is lowered with aging, is widely examined. In the present study, the effect of melatonin on characteristics of stress-induced mPTP opening in mitochondria isolated from young and old rats, treated or not treated with melatonin, has been examined. Oxidative stress was induced by 1 or 100 μM cumene hydroperoxide. It was found that chronic treatment of old rats with melatonin caused suppression of mPTP opening in mitochondria. Melatonin was found to be able to prevent the cumene hydroperoxide-induced swelling of mitochondria isolated from melatonin-treated rats.