We studied the peptide backbone modifications that improve the metabolic stability of the resulting peptides and yet retain high inhibitory activity against human plasma renin. A systematic investigation of N-alpha-methyl and C-alpha-methyl modifications at the P2 and P3 sites of renin-inhibitory peptides that contain part of the human angiotensinogen sequence led to the discovery of N-alpha-methyl amino acids at the P2 site as a useful structural modification. U-71,038 (11) inhibited human plasma renin with an in vitro potency (IC50) of 2.6 x 10(-10) mol/l. It is highly selective for renin and, as anticipated, resistant to proteolytic degradation. Additional study based on molecular graphic modelling has led us to propose a gamma-lactam conformational constraint at the P2-P3 site. This pseudo-dipeptide has proved useful in the preparation of active renin inhibitors. Compound 18a inhibited human plasma renin with an in vitro potency (IC50) of 2.1 x 10(-9) mol/l. This class of compounds also offers structural features for the study of enzyme-bound conformers.