Sodium-23 nuclear magnetic resonance was utilized to compare sodium binding to erythrocyte ghosts of normotensive and of essential hypertensive individuals. Plots of the reciprocal of the excess longitudinal relaxation rates as a function of total sodium ion concentration indicated tighter and more complex sodium interaction with erythrocyte membrane preparations from normotensives and a weaker, simpler sodium binding with membranes of hypertensives. NMR studies comparing 1) sodium-23 interaction with DIDS inhibition of chloride-35 interaction and 2) competitive effects of cations on the sodium interaction provided evidence for specific sodium binding to the cytoplasmic surface of the erythrocyte ghosts. The results are briefly considered relative to possible mechanisms for essential hypertension. © 1980, All rights reserved.