Regulin® implants contain melatonin, a natural secretion of the pineal gland. Melatonin is the day length messenger by which all animals recognise different seasons. The pineal gland only produces melatonin during hours of darkness. As days shorten the amount of melatonin secreted increases and this signals the reproductive system to increase activity, producing a natural peak in breeding performance in the autumn. Regulin works by mimicking this effect: each implant slowly releases melatonin over an extended period.