The design of a dialysate recirculating loop permitted assessment of dialysate/blood Na+ equilibrium which we established as: Na+ plasma water concentration of 143.4 mEq/l and Na+ dialysate concentration of 136.6 mEq/l (Donnan effect). Sodium and water removal using different Na+ dialysate concentrations have been assessed. With an Na+ dialysate concentration of 130 mEq/l plasma clearance of Na+ and H2O are equal, NNa/NH2o = 128–142. Using a dialysate Na+ concentration of 162 mEq/l, NNa/NH2o = 40–61, i.e. Na+ clearance is much lower than H2O clearance. The average patient requires removal of 2 liters H2O and 200 mEq Na+, i.e. NNa/NH2o = 100. This corresponds to an Na+ dialysate concentration of 145 mEq/l. The incidence of hypotensive episodes during dialysis will be reduced if NNa/NH2o ratios (Na+ dialysate concentrations) are individualized for hemodialysis patie