Rat renin cDNA was transfected into COS-7 and Chinese hamster ovary (CHO) cells and expressed under the control of the Simian Virus 40 early promoter. Conditioned media of the transfected cells showed renin activity only after trypsin treatment, suggesting prorenin was secreted into the medium. From the trypsinized serum-free culture of the transfected CHO cells active renin was purified to homogeneity by a simple three-step procedure. The active renin had similar specific activity, molecular weight, Km, pH optimum, and isoelectric point compared to native renin. The amino-terminal sequence was the same as that deduced from the renin cDNA. This suggests that the recombinant rat renin is similar to kidney renin in many respects, and is easily obtained by the present procedures.