Rat kidney renin was purified 29000-fold using a nine-step procedure with a 4% recovery. N-terminal sequence analysis of the non-reduced renin revealed two sequences, indicating a two-chain structure. When compared with the amino acid sequence deduced from the rat preprorenin cDNA sequence, the N-termini of the A and B chains were residues 72 and 355, respectively, of preprorenin. The two-chain structure was confirmed by sodium dodecyl sulphate polyacrylamide gel electrophoresis. Rat kidney renin migrated as two broad bands of 35000–36000 and 37000–38000 Da under non-reducing conditions. Under reducing conditions, the size of both bands decreased by approximately 3000 Da and a band migrated at the buffer front (∼ 5000 Da). The site of cleavage between the two chains of rat renin is analogous to that of mouse submandibular gland renin. However, processing of the prosequence of rat prorenin differs from that of mouse and human prorenin, indicating that the mechanism of activation of prorenin is species-specific.