The hallmark of aging is a progressive decrease in homeostatic capability — a decreased response to an internal or external environmental perturbation. The article by Phillips and associates* in this issue of the Journal provides a clear example of this phenomenon.Phillips et al. report that after a 24-hour period of restricted water intake, an equal weight loss occurred in a group of elderly male subjects and a group of young male controls. The older group had greater increases in plasma sodium osmolality and arginine vasopressin concentrations. Despite these higher concentrations, the elderly elaborated a less concentrated urine and experienced . . .