The effects of high sodium intake on erythrocyte ²²Na efflux rate constants were studied in 25 patients with essential hypertension and 9 normal subjects. With changes in sodium intake from 100 mEq to 300 mEq/day, both total and ouabain sensitive ²²Na efflux rate constants decreased significantly (p < 0.001) in “salt-sensitive” patients (-0.031 ± 0.005 and -0.035 ± 0.006 /hr, respectively), but these responses were variable in “nonsalt-sensitive” patients and in normal subjects. The “salt-sensitive” patients showed a significant increase in their body weight, while intraerythrocyte sodium contents remained unchanged in the both groups. These results suggest that the abnormal change in membrane Na-K-ATPase activity may, at least in part, be involved in the mechanism of sodium susceptibility in patients with essential hypertension.