1. An 800-fold increase in plasma renin occurs after aggressive behaviour in male mice without kidneys and submaxillary glands.

2. The aggression-provoked renin fulfils all the criteria so far studied for being identical with renin.

3. It is an acid proteinase which generates angiotensin I with the same Km and specific enzymatic activity as pure submaxillary renin and normal plasma renin.

4. It is immunologically identical with submaxillary renin in the direct renin assay and in crossed immunoelectrophoresis.

5. It is present in plasma as fully enzymatically active 40 000-mol. wt. renin.

6. It is not likely that it arises from an inactive precursor or binding protein in plasma, since there is no inactive renin present and the high-molecular-weight forms of renin are also present after the fighting.

7. None of many organs so far studied can account for the release.

8. Synthesis of renin de novo during fighting is not likely since it is released also after blockade in vivo of protein synthesis.

9. Origin of the renin is unknown.

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