1. In twenty-two patients representing different stages of benign essential hypertension, hyperosmotic saline was administered intravenously. Determinations of intra-arterial pressure, renal plasma flow, glomerular filtration rate and plasma renin concentration were carried out before and, in the majority, also during and after saline infusion. Changes in cardiac output were followed in ten patients. Plasma volume and extracellular volume were determined in the control period only, although haemodilution was assessed by haematocrit readings.

2. Excess of sodium excretion showed a wide range and was related to the patient's age, as well as to a set of parameters reflecting intrarenal pressure patterns; hypernatriuresis consistently occurred in older patients, in whom renal vascular resistance and nitration fraction were elevated and plasma renin was suppressed. It could not be clarified whether hypernatriuresis together with renin suppression were determined by intrarenal pressure relationships or by an independent age-related factor in the hypertensive patient.

3. Excess of sodium excretion was not related to increments in arterial pressure, cardiac output, renal blood flow or glomerular filtration rate.

4. Plasma renin concentration failed to show consistent changes after hyperosmotic saline infusion.

5. It is concluded that natriuresis is not mediated by changes in the activity of the renin-angiotensin system. Hypernatriuresis appears to be a feature of progressive benign hypertension.

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