Melatonin, the chief hormone of the pineal gland, is produced and secreted into the blood in a circadian manner with maximal production always occurring during the dark phase of the light:dark cycle. Whereas the 24h rhythm of melatonin production is very robust in young animals including humans, the cycle deteriorates during ageing. The rhythm of melatonin can be substantially preserved during ageing by restricting the food intake of experimental animals; this same treatment increases the life span of the animals. The exogenous administration of melatonin to non-food restricted animals also reportedly increases their survival. Moreover, melatonin has been shown to have immunoenhancing effects and oncostatic properties. The implication of these studies is that melatonin may have both direct and indirect beneficial effects in delaying ageing processes or it may retard the development of processes (e.g., immunodeficiency and tumor growth) which contribute to a reduced life span.