When infused intravenously into anaesthetized cats angiotensin II (1–4 μg/ kg) released into the circulation an unstable substance that caused de-aggregation of platelet clumps, relaxed a strip of bovine coronary artery, and its release was blocked by aspirin and indomethacin. Because of these characteristics this substance is likely to be prostacyclin. Catecholamines and phenylephrine did not induce the release of prostacyclin. It is suggested that a chemical modification of the molecule of angiotensin II may render a peptide with little hypertensive properties which will be an activator of prostacyclin biosynthes