Prolinal derivatives of the formula ##STR1## wherein A is a group of the general formula ##STR2## wherein one of R.sup.1 and R.sup.2 is a hydrogen atom and the other is an alkyl group of from 2 to 5 carbon atoms, alkoxy group of from 1 to 5 carbon atom(s), phenyl group, benzyl group, cycloalkyl group of from 4 to 6 carbon atoms or (cycloalkyl group of from 4 to 6 carbon atoms)--methyl group or R.sup.1 and R.sup.2 each represent the same or different alkyl group of from 1 to 4 carbon atom(s) or a group of the formula ##STR3## wherein m represents an integer of from 3 to 6, n represents an integer of from 3 to 10, D represents a specified heterocyclic ring which is unsubstituted or substituted. These derivatives possess inhibitory activity on prolyl endopeptidase and therefor are useful as treating and/or preventing agents for amnesia.
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