Despite the fact that melatonin has been released for public use in the United States by the Food and Drug Administration and is available over the counter nationwide, there currently is a total lack of information on the toxicology of melatonin. In Europe, melatonin has a completely different status in that it is considered a "neurohormone" and cannot be sold over the counter. Even though administration of melatonin in humans, as well as in animals (even at supraphysiological doses), has not shown evidence of toxicological effects (i.e., no deaths), a drug toxicological file still would need to be prepared and approved by the regulatory authorities. Several features that are specific to this neurohormone need to be taken into consideration. Whatever the species concerned, melatonin is secreted during the night; it is the "hormone of darkness." It presents a circadian rhythm and a circannual rhythm (in photoperiodic species). The duration of these secretions could have an impact on the reproductive system, for example, showing the importance of the pharmacodynamics of melatonin. An inappropriate time schedule of melatonin administration could induce supraphysiological concentrations of the neurohormone and a desensitization of melatonin receptors. A long duration of exposure to melatonin also could mimic an "artificial darkness" condition when a circadian rhythm with a basal zero level during the day needs to be conserved for a physiological function. Furthermore, administration of large doses of melatonin could induce high concentrations of melatonin and of different metabolites that could have deleterious effects per se. Numerous books, magazines, and articles have praised melatonin as a "miraculous cure-all" for ailments ranging from sleeplessness, to aging, without any clinical evidence of efficacy (with the exception of its chronobiotic and resynchronizing effect). Very little attention has been paid to the possible side effects of melatonin. Nightmares, hypotension, sleep disorders, abdominal pain, etcetera, have been reported. In fact, analysis of the known pharmacological profile of melatonin and/or of its metabolites, based on scientific preclinical studies, constitutes a basis for prediction of adverse drug reactions or side effects. These include (1) the central nervous system, (2) the cardiovascular system and platelet aggregation, (3) glucose metabolism, (4) immunology, and (5) cancer. The knowledge of the fundamental mechanism of action of melatonin, including molecular biology, also needs to be taken into account for evaluation of possible side effects. Two types of melatonin receptors have been cloned (related to cyclic AMP), and the possibility of intracellular action of melatonin cannot be excluded. Melatonin receptors are present in the periphery and also at the level of the central nervous system, particularly on the suprachiasmatic nucleus that "drives" a circadian rhythm to many other areas on which it projects. Among those, the hypothalamus (which has melatonin receptors) plays a fundamental role in the hormonal homeostasis and modulation control of the organism. Special preclinical and pharmacological studies that take into account all these parameters need to be designed for safety evaluation and risk assessment of this specific neurohormone.