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Melatonin is a hormone produced by pineal gland. As a synchronizer of endocrine system bioclock in vertebrate, melatonin possesses many physiological activities��e.g. regulating the biologic rhythm according to photoperiod, by its receptors widely distributed in pituitary gland, hypothalamus, optomeninx, kidneys, skin and its appendages, etc. In addition to central nervous system, genital system and immune system, melatonin is closely related to hair growth, and can affect the seasonal shift of animals' fur. So far, pilot trials have been conducted on the treatment of androgenetic alopecia with melatonin.