The widely accepted concept of body fluid and electrolyte homeostasis is that Na(+) is restricted mainly to the extracellular fluid and K(+) to the intracellular space, where both ions act to hold water and thereby control the extracellular and intracellular fluid volume by their osmotic activity. Na(+) accumulation thus inevitably leads to water retention. The constancy of the extracellular volume is the task of the kidneys, which control the total body Na(+) content. More recent data have questioned this traditional view, suggesting that large amounts of Na(+) can be accumulated without accompanying water retention by osmotically inactive Na(+) retention, or by osmotically neutral Na(+)/K(+) exchange. Besides the control of the body Na(+) content by the kidneys, redistribution of body electrolytes hence provides an extrarenal regulatory alternative in the maintenance of body fluid volume and blood pressure control.