SUMMARY Baseline plasma vasopressin concentrations were measured in 48 men (all 50 years old) with decreased plasma renin concentration and untreated, sustained essential hypertension and in 29 healthy normotensive men. Mean hypertensive plasma vasopressin concentration was more than twice as high as the corresponding normotensive level (15.7 ± 2.2 [SE] vs 7.5 ± 1.0 pg/ml; p < 0.001). Plasma renin concentration in the hypertensive group was reduced compared with that in the normotensive group (0.28 ± 0.04 vs 0.46 ± 0.06 Goldblatt units X 10 4 /ml). These differences appeared despite virtually identical serum osmolality, creatinine clearance, and urinary sodium excretion in the two groups. In the first 38 hypertensive subjects, arterial plasma epinephrine concentrations were significantly increased over those of the first 28 control subjects (99 ± 12 vs 68 ± 6 pg/ml; p < 0.025). In contrast to those with low renin essential hypertension, 35 men with normal renin essential hypertension (all 40 years old) had normal plasma vasopressin levels that were not significantly different from those in a comparable normotensive control group (3.7 ± 0.8 vs 3.5 ± 0.4 pg/ml). Arterial epinephrine concentrations were not significantly different between normal renin subjects and the control group. After 6 weeks of treatment with the nonselective /3-adrenergic receptor blocker oxprenolol in 11 subjects with low renin hypertension, blood pressure was reduced and the plasma vasopressin concentration fell from 27.6 ±6.4 to 13.5 ±4.2 pg/ml (p < 0.01). After 6 weeks of treatment with the /3,-selective agent atenolol in another 11 men with low renin hypertension, plasma vasopressin level fell from 16.3 ± 3.3 to 13.1 ±3.1 pg/ml, but this reduction was not significant. In pooled groups treated with /3-adrenergic blockade, plasma vasopressin level decreased more the higher its initial concentrations (r= 0.79, p< 0.001, n = 22). These findings suggest that plasma vasopressin is increased in low renin essential hypertension and may be related to sympathetic adrenal overactivity. (Hypertension 8: 506-513, 1986) KEY WORDS • blood pressure • catecholamines • sodium • osmolality • /3-adrenergic blockade • pituitary • antidiuretic hormone A RGININE vasopressin (AVP) may increase /\ blood pressure primarily by volume reten-A. \. tion, but it is also a powerful vasoconstrictive agent, even more potent than angiotensin.1 It may also participate in cardiovascular regulation, both centrally 2 and peripherally, 3 and stimulate the gain of baroreceptor reflexes. 4 Potentiation of the vasoconstrictor effect of catecholamines has been established. with mineralocorticoid excess. 6 -9 "" Deoxycorticosterone-salt hypertension will not develop in the Brattleboro rat, which lacks AVP because of a genetic defect. Blockade by either specific antiserum or competitive antagonists of AVP substantially attenuates both the development and severity of preexisting deoxycorticosterone-salt hypertension. Most recently, observations by Hay wood et al.12 point to the sy...