Amino acid derivatives of the formula ##STR1## wherein: Q is W--(CH.sub.2).sub.m --(A).sub.n --CO--B-- whereinA is oxygen atom or --NH--;B is ##STR2## or --(CH.sub.2).sub.l -- wherein X is sulfur atom or --CH.sub.2 --, and l is an integer from 1 to 3;W is ##STR3## wherein Hal is halogen atom; n is an integer of 0 or 1; andm is an integer from 0 to 3;Y is sulfur atom or --CH.sub.2 --; andR is hydrogen atom, formyl group, C.sub.1-5 alkoxycarbonyl group, --C.tbd.N or ##STR4## wherein R.sup.1 is hydrogen atom, andR.sup.2 is hydroxyl group, C.sub.1-5 alkylcarboxy group, phenylcarboxy group, C.sub.1-5 alkoxy group or phenyl C.sub.1-5 alkoxy group; with the proviso that when R is hydrogen atom, formyl group or C.sub.1-5 alkoxycarbonyl group, W is ##STR5## The compounds mentioned above specifically inhibit prolyl endopeptidase activity and are useful as agents for the treatment and prevention of dementia and amnesia.