1. Ten patients with essential hypertension received an intravenous load of hyperosmotic saline before and during treatment with propranolol. In seven patients blood pressure was lowered by the drug.
2. In the control period saline infusion induced a sharp rise in cardiac output due to an increase in stroke volume. This pattern was absent during beta-adrenergic blockade. Renal blood flow showed a moderate rise in the control studies; this rise was not reduced during treatment. Natriuresis was reduced by propranolol.
3. Measurements of central blood volume and central venous pressure in five patients revealed that modified responses during beta-adrenergic blockade may be due to a reduced venous return rather than to myocardial depression.