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.