1. Angiotensin I-generating activity of rat brain extract was separated into two components by affinity chromatography on a casein—Sepharose gel column.
2. The component without affinity to the gel was identified as true renin on the basis of its sensitivity to anti-renin antibody and the lack of protease activity.
3. The second renin-like component with affinity to the gel was a protease insensitive to the anti-renin antibody. Its renin-like activity examined with sheep substrate was pronounced compared with the rate of angiotensin I generation from the rat substrate.
4. It was concluded that rat brain contains true renin, which can be detected by the use of rat substrate but can be masked when examined with sheep substrate.