In a study of total body and erythrocyte potassium in mild hypertension we found decreased intracellular potassium concentrations in 41 hypertensives compared to controls but no correlation between intracellular potassium, measured by whole body counting, and erythrocyte potassium. A total body potassium corresponding to an intracellular potassium of 85% or less of the expected value was found in females. In the hypertensives, a negative correlation existed between serum and erythrocyte potassium. No correlation was found between potassium decrease and urinary aldosterone or plasma renin level. An inhibition of the active sodium-potassium exchange at the cellular level is proposed as an explanation of these findings.