Cell Na in the rat tail artery was measured by replacing all extracellular Na with Li at 2°C. Water was partitioned with ¹⁴C-sorbitol as the marker. The binding capacity of the cytoplasm was estimated by ion-exchange after the destruction of cell membranes by alternate freezing and thawing. Cell Na is of the order of 20–25 mEq/kg dry weight and about ²/₃ of this may be in free solution with [Na]_i in the range of 15 mEq/liter. Cell K is about 220 mEq/kg dry weight and, of this, 30 mEq or more is bound so that [K]_i is not greater than 190 mEq/liter. These estimates allow only for the low figure of 30 mEq as the sum of site-bound cell monovalent cations. It is shown, however, that as much as 50-60 mEq may be site-bound or otherwise restricted within the cell. A similar amount, mostly Na⁺, is site-bound within the paracellular matrix.