In order to clarify the possible relationship between changes in blood pressure after salt loading, membrane sodium transport and renin profile, 19 patients with essential hypertension (8 patients with low renin hypertension and 11 patients with normal renin hypertension), admitted to our hospital, were studied. We also examined the correlation of changes in intracellular sodium concentration after salt loading between erythrocytes and lymphocytes. After a control period of one week, all subjects were placed on a low salt intake for one week followed by one week of a high salt intake. Percent increases in mean blood pressure and intracellular sodium concentration in erythrocytes and in lymphocytes after salt loading were greater in low renin hypertensive patients than in normal renin hypertensives. Percent changes in intracellular sodium concentration in erythrocytes inversely correlated with those in ouabain sensitive sodium efflux rate constant and positively correlated with those in intracellular sodium concentration in lymphocytes. These results suggest that an increase in sodium chloride sensitivity of blood pressure in patients with low renin hypertension may be due to the inhibition of Na+-K+ pump in vascular smooth muscle cell membrane.