The synthesis of 1-[(2-mercaptocyclopentyl)carbonyl]-L-prolines, 1-[(2-mercaptocyclobutyl)carbonyl]-L-prolines and related benzoyl derivatives as pure isomers is described. The abilities of all the compounds to inhibit angiotensin converting enzyme (ACE) in vitro and in vivo and to lower the systolic blood pressure in renal hypertensive dogs were determined. Three of them, namely 1-[[2-(benzoylthio)cyclopentyl]carbonyl]-L-proline (10f(R,S], 1-[(2-mercaptocyclopentyl)carbonyl]-L-proline (10g(R,S], and 1-[[2-(benzoylthio)cyclobutyl]carbonyl]-L-proline (16f(R,S], were found to be as potent as captopril in reducing blood pressure. The influence of chirality and ring size on the ACE inhibition is described.