Sodium has a critical role in the regulation of weight, extracellular fluid volume, blood pressure and thirst. The sodium concentration in plasma water exceeds that in plasma by about 10 mmol/l, suggesting a large gradient driving sodium into the dialysate. However, most of this apparent gradient is eliminated by the effect of negatively charged plasma proteins that reduce the diffusible sodium. Hence, the plasma sodium concentrations is generally used instead of plasma “water” sodium as the relevant concentrations. This plasma sodium concentration may be a bit less, on average,compared with the diffusible, ionized sodium, and that the correlation between the diffusible sodium and the plasma sodium levels may differ a bit in individual patients. Sodium removal during haemodialysis can occur through convection or diffusion. Current prescribing practices for chronic intermittent hemodialysis rely primarily on convective losses (∼78%) and less on diffusive lossess (∼22%). Hypothetically, a regular removal of 1L of ultrafiltered plasma water, considering a theoritical isotonic sodium concentrations of 140 mmol/l in the ultrafiltrate, would be responsible for a removal of 140 mmol of sodium, equivalent to 8 g of sodium chloride ingestion in each interdialytic day. The usual dialysate sodium level is between 135 and 145 mmol/l. In general, a high-sodium dialysate would be above 141 mml/l, whereas below 137 mmol/l would be regarded as low sodium dialysate.