Renin-angiotensin-aldosterone system (RAS) is the most important blood pressure control system in animals (Fig. 1). Renin [EC 3. 4. 23. 15] is a highly specific aspartic proteinase that is mainly synthesized by juxtaglomerular (JG) cells in the kidney. The human renin gene encodes preprorenin consisting of 406 amino acids (1-23 signal sequence, 24-66 propeptide, and 67-406 mature renin) [Imai et al., 1983]. The synthesized renin precursor is processed to mature renin by proteolysis and stored in renin granules in JG cells. The secretion of renin into the circulation is controlled by several stimuli. The enzyme catalyzes the release of angiotensin I from plasma substrate angiotensinogen. This conversion is the rate-limiting step in RAS. Angiotensin I is an inactive peptide activated by angiotensin converting enzyme (ACE) [EC 3. 4. 15. 11]. ACE cleaves C-terminus dipeptide from angiotensin I. The angiotensin II produced acts directly on arterial smooth muscle cells to maintain blood pressure and stimulate the synthesis and release of aldosterone. Hence, RAS is a major target in the treatment of hypertension. ACE inhibitor is commonly used in clinical treatment. In connection with the control of renin activity, renin-binding protein (RnBP), a cellular renin inhibitor, was first isolated from porcine kidney as a complex of renin, called high-molecular-weight renin [Takahashi et al., 1983a, Takahashi et al., 1983b]. The nucleotide sequences of porcine, human, and rat RnBP cDNAs were determined and the amino acid sequences consisted of 402, 417, and 419 amino acid residues, respectively [Inoue et al., 1990, Inoue et al., 1991, Takahashi et al., 1994]. The co-expression of human renin and RnBP cDNAs in AtT-20 cells showed that RnBP regulates active renin secretion from the transfected cells [Inoue et al., 1992]. ACE has been used to screen inhibitors from foodstuffs because of its simple assay method, but renin is a rate-limiting enzyme in RAS, so it was not used because of the complicated assay system. In this chapter we describe expression of recombinant human renin in E. coli and Spodoptera frugiperda (Sf-9) insect cells, development of a simple and rapid assay method for human renin, occurrence of renin inhibitor in fermented soybean, isolation of renin inhibitors from soybean, and structure-function relationship of saponins.