An amino analogue of N-benzoyl-phenylalanyl-glycyl-proline, a tripeptide inhibitor of angiotensin-converting enzyme, was synthesized. The analogue (III) has the phenylalanyl-glycine amide linkage of N-benzoyl-phenylalanyl-glycyl-proline reduced to a methylene amine. Compound III was tested as an inhibitor of porcine plasma angiotensin-converting enzyme and has an I50 of 620 microM compared with an I50 of 9.6 microM for its parent tripeptide. These results are explained in terms of a proposed model of the converting-enzyme active site.