The synthesis and antihypertensive activity of a new series of N-(mercaptoacyl)-thiazolidinecarboxylic acids (VIIa-d) are described. Antihypertensive activity was evaluated in terms of angiotensin I-converting enzyme (ACE) inhibitory activity. The activities of these compounds were compared with that of (2S)-1-[(2S)-3-mercapto-2-methylpropanoyl] proline, SQ 14225, and many of them were found to be relatively potent inhibitors of ACE. The most potent was (4R)-2-(2-hydroxyphenyl)-3-(3-mercaptopropanoyl)-4-thiazolidinecarboxylic acid (62). Structure-activity relationships among the thiazolidines and some related compounds are discussed.