1. The sodium concentration within the erythrocytes of 159 subjects was found to be 7·35 ± 1·25 (SD) mmol/litre of cells.
2. In 157 normal subjects, the erythrocyte potassium concentration was 99·08 ± 5·3 (SD) mmol/litre of cells.
3. In the erythrocytes from twenty-seven normal subjects there was a striking linear correlation between the rate constant for active sodium efflux and resting sodium concentration.
4. It is concluded that these studies confirm the assumption that the resting concentration of sodium within a cell is determined by the activity of the sodium pump. What is not known is the mechanism by which this precise control is maintained.