1. The pressor responses to angiotensin I were compared with those to angiotensin II after injections into the left ventricle and jugular vein in the sheep, dog and pig.
2. The ability of angiotensin I to raise the blood pressure was less than that of angiotensin II with both routes of injection, a difference which was more marked after ventricular injection.
3. When equipressor doses of the hormones were given there was a delay of 1–3 s in the onset of the pressor response to angiotensin I compared with angiotensin II after left-ventricular injections; the difference in the delay in onset was less apparent with intravenous injections.
4. The development of the pressor responses was similar with both hormones when equipressor doses were used but the rises in blood pressure were more prolonged with angiotensin I, especially when given by the left-ventricular route.
5. The in vitro rate of activation of angiotensin I by blood was much slower than the apparent in vivo formation of angiotensin II.