[Present and future of melatonin in human and animal reproduction functions].

In the pineal gland, the photoperiodic information is translated into a chemical message, the nocturnal secretion of melatonin. The duration of this secretion is proportional to the length of the night and this is the way by which the photoperiodic information is integrated at the level of the central nervous system. It is now possible with melatonin to mime the effects of photoperiod on the reproductive axis. The daily melatonin secretion is directly generated in the circadian biological clock. Consequently, the periodic secretion of melatonin might be used as a circadian zeitgeber, able to entrain some circadian functions: it is thus possible to act with melatonin on circadian rhythms. At very large doses, melatonin seems to be able to act directly on gonads.