1-(3-Mercapto-2-methylpropanoyl)prolyl amino acid derivatives of the formula ##STR1## wherein R represents a hydrogen atom, a lower alkyl group, a phenyl-lower alkyl group or a substituted phenyl-lower alkyl group, R.sub.1 represents a hydrogen atom, R.sub.4 CO--, R.sub.5 S--, or ##STR2## R.sub.2 represents a hydrogen atom or a lower alkyl group, R.sub.3 represents a hydrogen atom, a phenyl group, a lower alkyl group, or a substituted lower alkyl group, or R.sub.2 and R.sub.3 form a heterocyclic ring together with the nitrogen and carbon atoms to which they are respectively bonded; salts of these compounds; compositions comprising these compounds or salts; and processes for preparing these compounds or salts. These compounds and salts have excellent antihypertensive activity.