A process for preparing perindopril (III) or a pharmaceutically acceptable salt thereof, which process comprises a substituted benzyl ester of (2S,3aS,7aS)-octahydroindole-2-carboxylic acid (I) with N--[(S)-carbethoxybutyl]-(S)-alanine (II) where R represents a halo, C.sub.1-4 alkoxy or nitro substituent.