A series of renin inhibitors containing the dipeptide transition state mimics (2S,4S,5S)-5-amino-4-hydroxy-2-isopropyl-7-methyloctanoic acid (Leu (OH)/Val) and (2S,4S,5S)-5-amino-4-hydroxy-2-isopropyl-6-cyclohexylhexanoic acid (CHa /(OH)/Val) was prepared. A structure-activity study with Boc-Phe-His-Leu (OH)/Val-Ile-His-NH2 (8a) as starting material led to N-[(2S)-2-[(tert-butylsulfonyl)methyl]-3-phenylpropionyl]-His-Cha (OH)/ Val- NHC4H9-n (8i) which has the length of a tetrapeptide and contains only one natural amino acid. Compound 8i had an IC50 of 2 x 10(-9) M against human renin and showed high enzyme specificity; IC50 values against the related aspartic proteinases pepsin and cathepsin D were (8 x 10(-6) and 3 x 10(-6) M, respectively). In salt-depleted marmosets, 8i inhibited plasma renin activity PRA and lowered blood pressure for up to 2 h after oral administration of a dose of 10 mg/kg.