Melatonin, the principal neurohormon of the pineal gland influences many physiological functions within the mammalians' body. The nocturnal peak of melatonin secretion may play a role in the regulation of the function of many organs and body systems (likewise the circadian rhythms and their functions), enabling them to maintain homeostasis and to adapt physiological functions to changing environmental conditions. New data suggests that melatonin may act directly on the water and electrolyte metabolism and renal function. It is possible that melatonin plays a role in the regulation of the functioning of the endocrine system through the nuclei in the hypothalamus, in order to inhibit synthesis and/or secretion of the pituitary vasopressin. Moreover, the presence of putative melatonin binding sites in the kidney suggests the possibility of the direct action of melatonin on the renal system. Melatonin may also indirectly affect the water and electrolyte metabolism through the influence on the behavioral effects.