1. Supine plasma renin activity and its responsiveness to erect posture and frusemide were reduced in fifty-one patients with essential hypertension, compared with fifty-one age- and sex-matched control subjects.
2. Twenty-four hour urinary sodium excretion was similar in hypertensive patients and control subjects, but after intravenous frusemide hypertensive patients excreted significantly less sodium.
3. A significant inverse relationship between plasma renin activity and diastolic blood pressure was demonstrated in hypertensive patients and in normotensive control subjects.
4. A significant inverse relationship between plasma renin activity and age, independent of blood pressure, was shown in hypertensive patients and control subjects.
5. It is concluded that the reduced renin values found in essential hypertension are, in part, the result of the elevated blood pressure acting on the kidney.