1. The major angiotensin I-generating activity of rat brain extracts has a pH optimum different from that of renal renin and is not inhibited by renin specific antibody.
2. Affinity chromatography utilizing renin specific antibody, pepstatin and α-casein yielded fractions that resembled renal renin more closely with respect to antibody inhibition and pH optimum as well as an absence of the ability to hydrolyse haemoglobin.
3. We conclude that rat brain contains a host of enzymes with angiotensin I-generating activity including acid and neutral proteases and an enzyme with the immunochemical identity of renal renin. The biosynthetic origins of this renin-like protein remain uncertain.