Fluid and electrolyte homeostasis depend on a balance between the intake and output of water. Aging is characterized by reduced homeostatic capacity. Changes in the control of both water intake and excretion accompany aging and may predispose the elderly to disturbances in sodium and water balance. Reduced thirst and water intake in response to water deprivation and thermal dehydration have been observed in healthy elderly persons. This reduction, combined with reduced renal water-conservation capacity, may predispose the elderly to dangerous dehydration when illness increases water losses or physical incapacity prevents access to water. The reasons for the thirst deficit are not clear. The elderly have a reduced capacity to excrete a water load, which means they are predisposed to water overload and hyponatremia. Furthermore, various neuroendocrine changes in the elderly affect fluid and electrolyte homeostasis. More studies are needed to understand the etiology of the disturbances of fluid intake and output so that they can be better prevented and treated.