Trifluoromethyl analogs of captopril, i.e (R,S)- and (S,S)-captopril-f , are synthesized and submitted to in vitro assay for inhibition of angiotensin converting enzyme ACE). It is found i' that (R,S)-captopriLf3 is substantially more potent than captopril (captopriLf3: ICjO = 2.9 x I@" iU). Stereoelectronlc and conformational fleets attributed to trifluoromethyl incorporation serve to explain the enhanced inhibitory activity.