Novel prolinal derivatives of the general formula: ##STR1## wherein A represents alkylene group of from 1 to 8 carbon atoms or alkenylene group of from 2 to 8 carbon atom(s) or a saturated hydrocarbon ring of from 3 to 7 carbon atoms, R represents hydrogen atom, phenyl group, benzyl group, alkyl group of from 1 to 8 carbon atom(s) or cycloalkyl group of from 3 to 7 carbon atoms,B represents alkylene group of from 1 to 8 atom(s) unsubstituted or substituted by phenyl group or benzyl group, or a single bond,D represents carbocyclic or heterocyclic ring unsubstituted or substituted by from one to three of halogen atom, alkyl or alkoxy group of from 1 to 4 carbon atom(s), nitro group or trifluoromethyl group which prolinal derivatives possess inhibitory activity on prolyl endopeptidase, and therefore are useful as a treating or preventing agent for amnesia.