The design and synthesis of new structural types of angiotensin converting enzyme (ACE) inhibitors, (R)-3-amino-4-oxo-2, 3, 4, 5-tetrahydro-1, 5-benzothiazepine-5-acetic acids (7, 26, 33 and 37) and (S)-3-amino-4-oxo-2, 3, 4, 5-tetrahydro-1, 5-benzoxazepine-5-acetic acids (8 and 27), are described. A number of compounds in these series showed potent ACE inhibitory activity in vitro and in vivo. The structure-activity relationship is also discussed.