In order to detect the portion of the angiotensin II molecule binding to the human arteriolar receptor, angiotensin II-(1-5)-pentapeptide, angiotensin II-(1-4)-tetrapeptide and angiotensin II-(1-3)-tripeptide were infused intravenously as saline solutions into the same 5 normal men from 0900 h at a rate of 30 nmol (20 micrograms)/kg.min (1.2 ml/min) for 15 minutes, at a rate of 90 nmol (54.5 micrograms)/kg.min (3.0 ml/min) for 6 minutes and at a rate of 140 nmol (54.5 micrograms)/kg.min (3.0 ml/min) for 6 minutes, respectively, on different occasions. At the end of the infusions average increases in blood pressure were 20/22 mmHg (p less than 0.001) and 6/7 mmHg (p less than 0.001), respectively, with the former 2 peptides, while the last peptide showed no pressor action at all. It had previously been found by our research group that angiotensin III, angiotensin II-(3-8)-hexapeptide, angiotensin II-(4-8)-pentapeptide, angiotensin II-(2-7)-hexapeptide, angiotensin II-(1-7)-heptapeptide and angiotensin II-(1-6)-hexapeptide have some pressor action but that angiotensin II-(5-8)-tetrapeptide has no pressor action in normal men. When these previous results are combined with the present result, it is evident that only tyrosine-containing molecules show pressor activity and that tyrosine-deleted molecules do not show pressor activity at all. It is concluded that tyrosine in position 4 is the key amino acid for the binding of angiotensin II to human arteriolar receptor.