1. Aortic homogenate contains renin-like activity which on incubation generates angiotensin I over a wide pH range.
2. Rat aortic renin measured at an incubation pH of 6·5 rose and fell in parallel to plasma renin with salt depletion and salt-loading respectively. Renin measured at an incubation pH of 5·3 showed little relationship with plasma renin.
3. Aortic renin (pH 6·5) was elevated in Goldblatt-two kidney hypertension and slowly fell for 24 h after bilateral nephrectomy whereas the fall in plasma renin was complete by the first hour. Aortic renin (pH 5·3) was also high, but did not fall after bilateral nephrectomy.
4. Aortic renin (pH 6·5) is probably derived from plasma renin whereas renin measured at pH 5·3 is probably a tissue renin.
5. The prolonged half-life of aortic renin (pH 6·5) explains the observation that the renin—angiotensin system appears to be active in maintaining blood pressure for several hours after bilateral nephrectomy whereas the decline in plasma renin is rapid and does not continue significantly beyond 1 h.