The structural characterization of idrapril, (1S,2R)-cis-2-[hydroxyaminocarbonylmethyl(N-methyl)-aminocarbonyl]cyclohexanecarboxylic acid (I), a novel ACE inhibitor and its related benzyloxy derivative (II) has been carried out by NMR studies.The crystal structure of II has been determined by x-ray diffraction.The NMR spectra indicate the presence of two cis and trans isomers with respect to the amide bond at room temperature, with rotational barriers of 70 kJ mol-1.The preferred conformation of these compounds is the trans rotamer with the carboxylic moiety in the equatorial orientation.Conformational studies in aqueous solution have been reported for idrapril as a function of pH and the equilibrium constant has been determinedThe results suggest the presence of intramol. hydrogen bonds, as is confirmed by mol. mechanics calculations