Abnormalities in cellular sodium metabolism have long been thought to play a role in the pathogenesis of hypertension. Techniques for measuring membrane electrolyte transport rates have been refined, providing an opportunity to characterize sodium metabolism in cells from hypertensives. The early clinical studies examined erythrocytes and leukocytes because of their easy accessibility and the possibility that they would reflect vascular tissue metabolism, and documented differences in sodium fluxes between hypertensives and normotensives. Subsequent studies have supported the early findings that abnormalities in membrane cation transport were associated with essential hypertension.