Because several well-studied strains of rats manifest spontaneous hypertension, we set out to design a renin inhibitor suitable for use in this species. On the basis of the sequence of the renin substrate, a series of substrate analogue inhibitory peptides were synthesized by systematically modifying the P5, P3, P2, P1P1', P2', P3', and P4' positions. In assays against rat plasma renin, we found that modifications at the C-terminal segment have a marked influence on potency, and that a secondary butyl side chain at the P2' position is important for obtaining optimal activity. The structure at the P3' position, however, could vary considerably without significant effect. The steric effect of the P2 position was important; there an isopropyl side chain provided optimal binding between the inhibitor and the enzyme. At the P3 and P5 positions, potency appeared to depend on aromatic side chains. The effects at the P1P1' position of the transition-state residue (3S,4S)-4-amino-3-hydroxy -6-methylheptanoic acid (statine) and its congeners (3S,4S)-4-amino-3-hydroxy-5-phenylpentanoic acid (AHPPA) and (3S,4S)-4-amino-3-hydroxy-5-cyclohexylpentanoic acid (ACHPA) were found to depend on the sequence of the C-terminal segment. For peptides with an unfavorable C-terminal segment (-Ile-Phe-NH2), AHPPA and ACHPA resulted in a surprising retention of potency. For peptides with a favorable C-terminal segment (-Leu-Phe-NH2), the effect of AHPPA was mild, even though ACHPA still significantly enhanced potency. The hypotensive and plasma renin inhibitory effects of three of the analogues were then studied in anesthetized sodium-depleted rats. One of the compounds, acetyl-His-Pro-Phe-Val-Statine-Leu-Phe-NH2 (IC50 against rat plasma renin of 30 nM at pH 7.4), proved to be a potent hypotensive agent and a potentially useful probe for the study of the renin-angiotensin system in rats.