1. The exchange of potassium by the human erythrocyte has been studied in vitro using radioactive potassium.

2. An incubation technique which maintains erythrocytes in an essentially normal state for over 48 hours was employed.

3. Exchange of radioactive potassium between the red cells and the extracellular fluid was regular and progressive, the specific activities of the intra- and extracellular fluids reaching equal values. This indicates that all the erythrocyte potassium is exchangeable and is exchanging at the same rate.

4. From these data, it was calculated that at 37°C., 1.6 per cent of the erythrocyte potassium exchanges per hour, corresponding to an exchange of 1.5 mM of potassium per liter of red cells per hour. The time required for the exchange of 50 per cent of the red cell potassium is calculated to be 43 hours.

5. The temperature coefficient (Q10) of the potassium exchange rate is 2.2. This is the same as the temperature coefficient of the rate of utilization of glucose by the human erythrocyte.

6. Varying the percentage of red cells, plasma potassium concentration, initial glucose level, and pH between 7.0 and 7.7 had no effect on the potassium exchange rate.

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