In general, because of its higher specific activity 2-I125-iodomelatonin allows a lower detection limit thus allowing the use of a smaller amount of sample. melatonin and indicated the clinical purpose were chosen for inclusion in the review. == Data Extraction == The authors critically evaluated the methodological issues associated with numerous tools and techniques (e.g. requirements, protocols, and procedures). == Results of Data Synthesis == Melatonin measurements are useful for evaluating problems related to the onset or offset of sleep and for assessing phase delays or improvements of rhythms in entrained individuals. They have also become an important tool for psychiatric diagnosis, their use being recommended for phase typing in patients suffering from sleep and mood disorders. Additionally, there has been a continuous interest in the use of melatonin as a marker for neoplasms of the pineal region. Melatonin decreases such as found with aging are or post pinealectomy can cause alterations in the sleep/wake cycle. The development of sensitive and selective methods for the precise detection of melatonin in tissues and fluids has increasingly been shown to have direct relevance for clinical decision making. == Conclusions == Due to melatonins low concentration, as well as the coexistence of numerous other compounds in the blood, the routine determination of melatonin has been an analytical challenge. The available evidence indicates however that these challenges can be overcome and consequently that evaluation of melatonin’s presence and activity can be an accessible and useful tool for clinical diagnosis. Keywords:Melatonin, Circadian rhythms, Radioimmunoassay, Enzyme-linked immunoassay, High-performance liquid chromatography, Mass spectrometry, Capillary electrophoresis == Introduction == Melatonin (N-acetyl-5-methoxytryptamine) is usually a compound secreted mainly by the pineal gland, but Dehydrocholic acid synthesized also in many other tissues and cells, including the retina [13], human and murine bone marrow cells [4], platelets [5], the gastrointestinal tract [6], skin [7], or lymphocytes [8]. Because of the multiplicity of sites of formation and the presence of melatonin receptors in diverse locations, melatonin appears to be a versatile physiological signal that has been found involved in the control of numerous physiologic processes [9,10]. In mammals, photoperiodic information is usually relayed through the secretion of melatonin by the pineal gland, which then acts on the brain, and the neuroendocrine system to produce adaptive changes in endocrinology, anatomy, and physiology, thus affecting sleep, reproduction, molting, immune responses, energy balance, and behavior, among others [1114]. Moreover, melatonin exhibits direct and indirect antioxidant properties, and there is strong evidence that this compound can counteract the deleterious effects of reactive oxygen and nitrogen species in different systems [1519]. Although numerous physiological functions have been attributed to melatonin, the mechanisms involved in such functions are frequently unclear, especially when parallel signaling pathways that are initiated via the membrane receptors MT1or MT2are considered, or when other melatonin binding sites are involved [20]. Therefore, further investigation at cellular and molecular Dehydrocholic acid levels is needed to elucidate how this compound really functions as a relevant physiological regulatory transmission. Although in vitro studies can furnish important information on the effects of melatonin in cell culture or perfused tissues, studies of the fluctuations of melatonin concentration in body fluids and tissues generally have the most relevance for understanding its function in organisms. Thus, the development of sensitive and selective methods for the precise detection of these compounds in tissues and fluids of animals has become necessary. However, due to its low concentration and Dehydrocholic acid the coexistence of many other endogenous compounds in blood, the determination of melatonin has been an analytical challenge. This review discusses current methodologies employed for detection and quantification of melatonin in biological fluids and tissues. == Melatonin fluctuations in organisms == In vertebrates, the chronobiologically relevant portion of melatonin is mainly produced and released into the blood circulation by either the pineal gland, especially in mammals, or pineal gland plus retina, e.g., in some birds and amphibians [21,22]. The pineal glands of non-mammalian species are directly photosensitive while those of mammals are controlled by neuronal phototransduction pathways originating in the retina and then processed by the hypothalamic circadian pacemaker, the suprachiasmatic nucleus (SCN) [21,23,24]. In ICAM1 various birds, light influences circadian oscillators present in the pineal gland and acts additionally on neuronal pathways of retinal or hypothalamic origin modulating the gland [2527]. The contribution of extrapineal/extraretinal melatonin to blood plasma concentrations of the hormone are either very low or, in the case of gastrointestinal origin, only episodic and without profound chronobiological significance [6]. In humans, nocturnally peaking high-amplitude oscillations of melatonin in plasma are paralleled by corresponding variations in.