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N-Mercaptoalkanoyl derivatives of sulfur-containing amino acids were synthesized and examined for inhibitory effects on angiotensin I converting enzyme (ACE) extracted from rabbit lung. Inhibition of ACE was determined by means of a spectrometric assay with hippuryl-Lhistidyl-L-leucine as a substrate. Among the synthesized sulfur-containing compounds, N-(2-benzy1-3-mercaptopropanoy1)-S-methyl-L-cysteine (13a) and N-(2-benzy1-3-mercaptopropanoy1)-S-ethyl-L-cysteine (13c) showed the most potent inhibitory effects on ACE activity. The IC50 values of 13a and 13c on ACE activity were 0.028 and 0.020 pm, respectively.