Total exchangeable sodium and plasma volume were measured to evaluate role of sodium in hypertension by radioisotope 24 Na and Evans blue in 25 patients with essential hypertension, 10 with malignant hypertension, 5 with renal hypertension and 15 normal controls. The results obtained are as follows. 1. The total exchangeable sodium content was 35.8+/-3.5mEq/kg in normal controls, 35.6+/-2.7mEq/kg in essential hypertension and 36.2+/-3.5mEq/kg in renal hypertension revealing no statistical significance in difference. In malignant hypertension, it was 44.9+/-2.0mEq/kg and it was markedly elevated than in normal controls. 2. Plasma volume in normal controls was 44.0+/-4.7ml/kg. In essential hypertension there was two groups, one (group II, 35.2+/-5.6ml/kg) was slightly lower than the other (group I, 43.2+/-4.8ml/kg). In malignant hypertension, it was 56.9+/-7.5ml/kg revealing significant increase than in normal controls. 3. Changes of the total exchangeable sodium content was observed after the administration of hydrochlorothiazide 50mg in normal controls and essential hypertension. In normal controls there was no statistical significance in reduction. In essential hypertension, initially low salt group (group II) showed no significant reduction and the other group (group I) showed significant reduction with depression on blood pressure. 4. Long term therapy with hydrochlorothiazide 50mg for 2~3 months in the group of hypertension who responded well with short term therapy (group I), the contents of exchangeable sodium reduced significantly than normal controls and the antihypertensive effect was sustained. 5. The reduction of plasma volume with hydrochlorothiazide in essential hypertension are similar as changes of total exchangeable sodium. 6. In cases of low salt diet 4gm of daily salt intake, there was similar results as diuretic therapy in exchangeable sodium. 7. In patients who responded well to antihypertensive effect of hydrochlorothiazide or low salt diet (group I), salt loading of 10gm daily showed an increase of total exchangeable sodium and an elevation of blood pressure with statistical significance.