A few milligrams of a novel indoleamine were initially extracted from 250,000 ovine pineal glands, and the extracted molecule was structurally identified as N-acetyl-5-methoxytryptamine in 1958. It was given the common name melatonin because it is a methoxy derivative of serotonin and, in amphibians, it has a regulatory influence on melanin dispersion in epidermal melanocytes. In mammals, it was initially widely investigated for its effects in determining annual fluctuations in reproductive competence in seasonally breeding mammals. In the last two decades, however, the documented functional implications of this secretory product haveexpanded markedly. It is possible, considering its known receptor and nonreceptormediated actions, that melatonin may well influence the physiology of every cell inthe organism.