1. Physicochemical properties of renin secreted by isolated perfused rat kidney were examined and the results compared with those obtained for the renin in renal extract.

2. In renal extract, two high-molecular-weight reruns (molecular weight 65 000 and 55 000) and one low-molecular-weight renin (molecular weight 39 000) were found. Their relative proportion varied depending on extraction conditions. By acidification, high-molecular-weight renins were converted into low-molecular-weight renin without marked changes in activity.

3. In renal perfusate only low-molecular-weight renin was found after renin stimulation by isoprenaline or anoxia. Inactive renin was not found.

4. Renin in renal extract and perfusate samples were both found to consist of at least four isoenzymes having different isoelectric points (pI). The pI patterns were identical in renal extract and perfusate samples: pI 5.7 (60-70%), 5.5 (15-25%), 5.3 (5-10%) and 5.0-5.2 (2-5%).

5. These results indicate that the native renin secreted by rat kidney consists entirely of the low-molecular-weight and active form comprising multiple isoenzymes with a stable pI pattern.

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