1. In thirty-nine normotensive and hypertensive human subjects plasma concentrations of angiotensin II during pressor infusions of the peptide were significantly related to the concentrations found before infusion began. 2. When mean diastolic pressure before and during lower rates of angiotensin infusion were plotted against concurrent plasma angiotensin II concentrations, a straight-line relationship was observed. This relationship also held for the higher rates of infusion in some subjects. 3. During infusion at a standard rate of 4 ng min−1 kg−1 the rise in plasma angiotensin II concentration varied widely from subject to subject. 4. There was a significant negative relationship between this increase and the threshold of the pressor response to angiotensin. 5. There was a significant positive relationship between the threshold of the pressor response and the basal concentrations of renin and angiotensin II. 6. These results suggest that angiotensin normally present in blood lies within or close to a range capable of affecting blood pressure.

No abstract

1. The changes of peripheral venous plasma renin concentration (PRC) induced by head-up tilting were studied in four patients with orthostatic hypotension. 2. Two of the patients had the Holmes—Adie syndrome and tests of autonomic function suggested that they had an afferent block from baroreceptors with intact efferent pathways; the others had no evidence of the Holmes—Adie syndrome and investigations suggested that they had interruption of efferent sympathetic pathways. 3. In the two patients in whom lesions of the afferent side of baroreceptor reflexes were suspected, a marked increase in PRC occurred with upright tilting, whereas no change in PRC occurred in the two patients thought to have an efferent sympathetic block. 4. During repeated tilting, supine blood pressure and PRC increased progressively in the two patients with suspected afferent block, but not in the two patients with suspected efferent block. 5. It is suggested that an increase in plasma renin may contribute to the supine hypertension sometimes observed in patients with orthostatic hypotension. 6. It is also suggested that renin release does not require intact autonomic reflexes although certain components of efferent sympathetic pathways, not dependent on baroreceptor reflexes, may be important.