[Melatonin and aging].

Melatonin is a hormone secreted mainly by the pineal gland but also by the retina. It is synthesized from tryptophan and its characteristic circadian rhythm is ruled by light through the control of two limiting enzymic activities N-acetyl-transferase and hydroxyindole-O-methyl-transferase. In man--as in all studied species--its maximum plasma concentration is at night. Therefore it is considered as a signal transducing information on solar light within the organism thus providing the temporal framework upon which metabolic pathways are organized. Since its circulating levels decrease with ageing the question arises as to the origin and/or the consequences of this decrease, as well as the possible beneficial effects which could result from its administration in ageing humans.