We compared the ionic activity of sodium, as measured with glass electrodes, with sodium concentration in 23 healthy persons, 15 persons with acute renal failure, and before and after dialysis in 46 patients with chronic renal failure. In healthy persons the mean (± SEM) sodium concentration was 139.1 ± 0.6 mmol/L, whereas the ionic activity was equal to that of a 145.2 ± 0.5 mmol/L solution of sodium chloride. Variation in the concentration of plasma protein was the most important factor influencing the sodium activity coefficient (the ratio between activity and concentration). The sodium activity coefficient in plasma water (corrected for the non-aqueous phase of the plasma) was fairly constant, being 96% of that in a 140 mmol/L solution of sodium chloride. Thus sodium binds to non-protein molecules and sodium ions interact with other substances in uremic plasma only to a very limited extent. The sum of the molar activities of sodium, potassium, urea, and creatinine was closely and linearly correlated with plasma osmolality, both before and after dialysis.