In health, both the total amount and the distribution of water and electrolytes in the body are controlled within narrow limits. While intercompartmental water fluxes depend solely on physical driving forces, the latter are determined by complex circulatory feedback mechanisms that manipulate capillary pressure, and by active transport mechanisms that affect intracellular osmotic pressure. Fluid and electrolyte intake are determined by central neural mechanisms. However, the fine tuning of body fluid and elctrolyte contact is accomplished by alterations in renal excretion controlled by a variety of local (intrarenal) mechanical and endocrine processes, as well as external neural endocrine feedback systems that include the antidiuretic hormone and the renin-angiotensin-aldosterone systems. Analysis of renal control is complicated by multiple interactions among the different control systems.