In 22 50-year-old men with long-standing, untreated essential hypertension of the low renin type, venous plasma vasopressin concentrations were about three times those of 15 matched normotensive control subjects (p less than 0.005). These patients also had increased arterial concentrations of noradrenaline and adrenaline (p less than 0.05) but there was no direct association between these two catecholamines and vasopressin. On the other hand, adrenergic beta-receptor blockade with oxprenolol reduced both blood pressure and plasma vasopressin (p less than 0.01) while venous plasma dopamine concentrations significantly increased. In addition, the hypertensives had highly significantly increased serum uric acid (p less than 0.001) that correlated positively with venous vasopressin concentrations (p less than 0.05). According to these data, patients with the volume-sustained low renin type of essential hypertension have increased plasma vasopressin concentrations that probably are inversely related to dopaminergic nervous activity. The data also indicate that increased plasma vasopressin correlates with serum uric acid, most probably through increased tubular reabsorption of this acid.