Abstract: New peptides represented by the following general formula (wherein X is OH or NH2, R is an amino acid residue selected from Arg, His, Lys, Methyl-Arg or Methyl-Tyr, n is an integer of 1-4, and in the case that n is 2-4, R may be identical with or different from each other) and an anti-dementia drug which contains one or more of these new peptides and has a high blood brain barrier (BBB) permeability and furthermore is low in a side effect.

Abstract: New thiazolidine derivatives of the general formula: ##STR1## wherein X stands for a proline or thioproline residue and R for --CH.sub.2 OH or --CHO, a process for preparing the same which comprises reacting N-benzyloxycarbonyl-L-proline or --L-thioproline with L-thioprolinol and, if necessary, oxidizing the resultant N-benzyloxycarbonyl-L-prolyl-L-thioprolinol or --L-thioprolyl-L-thioprolinol to the corresponding thioprolinal compound, and an anti-amnestic composition containing at least one of the thiazolidine derivatives together with a conventional excipient. The new thiazolidine derivatives are extremely strong in anti-amnestic activity when compared with known similar anti-amnestic agents.

Abstract: New proline derivatives of the general formula: ##STR1## wherein R stands for a lower alkyl group and X for a N-terminal protective group conventional in amino acid chemistry or an acyl group derived from an amino acid with a protective group at the N-terminal. They have inhibitory activity against prolylendopeptidase and are relatively chemically stable as compared with known compounds of this type.

Abstract: An anti-amnestic agent which comprises as an active ingredient thereof at least one prolinal derivative of the general formula: ##STR1## wherein A is a protective groups for amino group in the field of amino acid chemistry and X is a residue of an amino acid, in a pharmacologically active amount together with an exipient. This agent is less toxic and can be safely administered to patients orally or intravenously for the remedy of memory disorder.