1. The biosynthetic precursor of renin (pre-prorenin) from mouse kidney is a single chain polypeptide with a molecular weight of 50 000. 2. This is the same value as previously found for mouse submaxillary gland pre-prorenin. 3. Mouse kidney pre-prorenin (mol. wt. 50 000) is larger than the enzymatically active renin (mol. wt. 40 000).