1. A radioimmunoassay for angiotensin I is described.
2. An extraction procedure for angiotensin I from blood was developed, using alcoholic precipitation to stop the renin/renin substrate reaction.
3. The normal range found for angiotensin I in venous blood was 11–88 pg/ml and in arterial blood 12–72 pg/ml.
4. Angiotensin I levels fell following bilateral nephrectomy but angiotensin I was still detected several weeks following operation.