18-hydroxy 11-deoxycorticosterone (18-OH DOC), a weak mineralocorticoid, was estimated by a radioimmunoassay procedure after purification in 49 patients with hypertension and 38 normal control subjects. The sensitivity of the method was 2-4 pg; there was no detectable blank, and the precision was 9-10%. In normal subjects the absolute plasma levels were similar to those of aldosterone. ACTH administration produced a 23-fold increase, and sodium restriction resulted in a 4-fold increase (5.4+/-0.7-20.5+/-3.0 ng/dl). On the other hand, the plasma levels of 18-OH DOC declined by nearly 50% with upright posture or angiotensin II infusion. During both of these procedures, plasma aldosterone levels significantly increased. Patients with normal and low renin hypertension had similar changes in plasma 18-OH DOC levels with sodium restriction. However, the mean high sodium level in the normal renin essential hypertension group (11.6+/-1.6 ng/dl) was significantly greater (P is less than 0.001) than in the control group (5.4+/-0.7 ng/dl). In addition, at least 22% and perhaps as high as 37% of the hypertensive subjects had levels greater than the upper limits of normal on a high sodium intake. Differences between the groups were less impressive in the sodium-restricted studies. There were no significant differences in age, duration of hypertension, sodium balance, serum sodium, potassium, or blood urea nitrogen in those patients who had elevated levels of plasma 18-OH DOC. Patients with primary aldosteronism had levels within the normal range on both dietary intake. However, in contrast to the other groups there were no significant changes in the plasma levels with sodium restriction. Thus, a significant number of patients with essential hypertension presumably have an alteration in 18-OH DOC secretion.