1. Antibodies were raised in rabbits against pure rat angiotensinogen. The antisera obtained were highly specific for rat angiotensinogen and did not bind hog, dog, rabbit, monkey or human angiotensinogen. They did not cross-react with angiotensin I, angiotensin II or synthetic hog tetradecapeptide renin substrate. However, rat des-angiotensin I—angiotensinogen cross-reacted 100% with the angiotensinogen antibody.

2. A direct radioimmunoassay for rat angiotensinogen in plasma was developed and this enabled 5 fmol of this protein to be detected. Comparison of the amounts of angiotensinogen determined by the indirect and direct assay systems indicated a 1:1.2 ratio for normal rats and rats in various endocrine states, except for adrenalectomized animals. In the latter, the angiotensinogen level measured by direct radioimmunoassay was four times that obtained by indirect assay.

3. The presence of a large amount of des-angiotensin I—angiotensinogen in adrenalectomized rat plasma is discussed.

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