Melatonin assays in body fluids.

A variety of methods now exist for the assay of melatonin in body fluids. Their relative merits are compared and the validation of one in particular (RIA) described. Physiological studies of melatonin by RIA have shown probable modulation of its secretion by gonadal steroids. The circadian activity maximum in the dark phase of one of the pineal melatonin synthesizing enzymes, N-acetyltransferase, is reflected in peripheral melatonin levels. Man, like all other species studied so far, has a dark phase rise in circulating melatonin. During the menstrual cycle, melatonin shows a luteal phase rise. Further evidence of pineal rhythmicity is found in seasonal melatonin variations in man. The study of the rhythmic properties of peripheral melatonin in man may provide important information on central nervous function.