alpha-[6-[[(S)-1-(Ethoxycarbonyl)-3-phenylpropyl]amino]-5-oxoperhydro -1,4-thiazepin-4-yl]acetic acids (monoester monoacids) and their dicarboxylic acids having the hydrophobic substituents at the 2- or 3-position of the thiazepinone ring were prepared and assayed for angiotensin-converting enzyme (ACE) inhibitory activity. The dicarboxylic acids having the pseudoequatorial amino groups at the 6-position and the pseudoequatorial hydrophobic substituents at the 2- or 3-position of the chair conformation of the thiazepinone ring had potent in vitro inhibitory activity. The monoester monoacids having the hydrophobic substituents at the 2-position suppressed pressor response to angiotensin I for a longer duration than those having the substituents at the 3-position when administered orally. The structure-activity relationship was studied by conformational energy calculations of the thiazepinone ring.