In rats administration of renin decreased plasma angiotensinogen and increased plasma renin, while administration of angiotensin had the opposite effects. However, both agents stimulated angiotensinogen formation by the liver. The stimulating effect of angiotensin II, and to a lesser extent of angiotensin I, was also demonstrated by adding these peptides to the perfusate of an isolated liver preparation. We propose a positive feedback effect of circulating angiotensin on angiotensinogen formation and release.