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.