The quantities of body fluids and electrolytes, as well as the osmolality, are controlled within tight boundaries. It is generally thought that total body water (TBW) is essentially determined by total body sodium (TBS) and total body potassium (TBP), and that TBS basically determines extracellular and plasma volume, thus, TBS is a major determinant of arterial blood pressure (BP). Balance studies of 4 days duration in freely moving dogs addressed the questions (i) to which extend BP is determined by TBS and TBW, and (ii) whether Na⁺ is usually stored in an osmotically inactive form during increase of TBS, as recently suggested. Alterations in TBS that covered the range from moderate deficit to large surplus were induced by 10 protocols. With normal or elevated TBW, individual BP changes correlated well with TBW changes, whereas, at lowered TBW, no correlation was found. Changes in TBS were very often accompanied by changes in TBP, and the sum of individual changes in TBS and TBP correlated very well with TBW changes; it was always accompanied by osmotically adequate TBW changes. Calculations reveal that even moderate TBS changes often included substantial Na⁺/K⁺ exchanges between extracellular and cellular space. Conclusions: (i) with increased TBW, TBW is a major determinant of BP, (ii) osmocontrol effectively adjusts TBW to the body's present content of both major cations, Na⁺ and K⁺, (iii) cells may serve as readily available, osmotically active Na⁺ store, (iv) the mechanism(s) that connect external Na⁺/K⁺ balances with compartmental Na⁺/K⁺ exchange are yet unknown.