Melatonin, synthetized by the pineal gland, is the chemical messenger which allows seasonally reproductive animals to perceive day length changes. In the ewe, the neural message, transformed into a hormonal one, triggers pulsatile activity of the LHRH neurons. About 40 days are necessary for melatonin to centrally stimulate the pulsatile LHRH activity. Its sites and mode of action are not yet known completely, but a precise hypothalamic zone has been defined in which radioactive melatonin binds specifically and where cold melatonin delivered locally stimulates LHRH activity. In the veterinary clinic, the most frequent mode of distribution is a sub-cutaneous implant, which induces an advancement of the cyclical ovulatory activity of ewes and goats. The date of fertilization is advanced and fecundity of females is improved. It can be used alone, or in association with other hormonal treatments, or after an artificial photoperiodic treatment. Under these conditions, it allows a quantitative and qualitative increase in out-of-season sperm production in rams and billy-goats. Such an implant is registered and marketed in France, the UK, Greece, Australia and New Zealand.