ABSTRACT
Two major physiological functions have been established for the pineal hormone melatonin (Af-acetyl-5-methoxy tryptamine) in vertebrate species. First, melatonin is important for maintaining circadian organization in some species, while more subtle effects of melatonin on circadian organization have been demonstrated in other species (for reviews, see 1-12). Second, melatonin is critical for the regula tion of reproduction in many seasonally breeding mammals (12). Other roles have been proposed for circulating melatonin, but these (e.g., effects on the immune system; local actions in gut and retina; 13,14) are less firmly established.
