Leucocyte sodium and potassium content and concentrations were measured along with ouabain-sensitive and ouabain-insensitive rate constants for sodium efflux in 14 controls and 20 black patients with essential hypertension. Leucocyte sodium content was significantly increased in the patients (mean 101-1±7-8 mmol/kg dry solids v 745 + 76 mmol/kg dry solids; p <0 05), whereas the rate constants for sodium efflux were not significantly reduced. There was no difference between the two groups in

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... potassium values. The increase in leucocyte sodium content in the presence of normal rate constants for sodium efflux suggests an increase in membrane permeability to sodium, which might be important in the pathogenesis of essential hypertension.