1. Intravenous injections of pure angiotensin I and angiotensin II give similar blood pressure responses in the rat, rabbit and guinea-pig although angiotensin I is generally regarded to be the inactive precursor of angiotensin II.
2. The conversion of angiotensin I to angiotensin II and of phenylthiocarbamyl (PTC)-angiotensin I to PTC-angiotensin II in human blood and plasma was studied using the rat colon as the assay tissue.
3. The results suggest that the rate of conversion of angiotensin I to angiotensin II in blood and plasma is insufficient to explain the comparable activities of angiotensin I and angiotensin II in raising the blood pressure.