Abstract: A prolinal derivative of general formula: ##STR1## [wherein A represents alkylene group of from 1 to 4 carbon atom(s) or saturated hydrocarbon ring of from 3 to 7 carbon atoms,n represents a number 2, 3 or 4,D represents carbocyclic or heterocyclic ring unsubstituted orsubstituted 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.]. possess inhibitory activity on prolyl endopeptidase, and therefore be useful for treating and/or preventing agent for amnesia.