Peptide derivatives and antidemetia agents

A peptide derivative having the formula (I):

• ##STR20## wherein A between Pro and Q.sup.2 is Arg or Lys:
• Q.sup.1 is pGlu or H;
• Q.sup.2 is -Gly-OH or OH;
• Y.sup.1 is H or --CO--T and Y.sup.2 is OH or T;
• wherein
• T represents a group having the formula (II):
• ##STR21## wherein R.sup.1 is a group selected from the group consisting of an alkylcarbonyl group having 2 to 7 carbon atoms, an arylcarbonyl group having 7 to 10 carbon atoms, and an alkylthio group having 1 to 6 carbon atoms;
• or
• a group having the formula (III):
• ##STR22## wherein R.sup.2 is a group selected from the group consisting of hydrogen, an alkylcarbonyl group having 2 to 7 carbon atoms, and an arylcarbonyl group having 7 to 10 carbon atoms;
• wherein at least either Y.sup.1 is --CO--T, or Y.sup.2 is T,
• n is 0 or 1;
• and amino acids constituting the peptide are those of type L with the proviso that both Pro and Arg are of type D;
• or functional derivatives of said peptide derivative selected from the group consisting of N-acyl derivatives having a group of --NHCOR wherein R is an alkyl group of 1-5 carbon atoms, a derivative in the form of amide, a monoalkyl-substituted amide or dialkyl-substituted amide derivative respectively, represented by --CONH.sub.2, --CONHR or --CONR.sub.2 wherein R is an alkyl group of 1-6 carbon atoms, or a derivative in the form of ester represented by --COOR wherein R is an alkyl group of 1-18 carbon atoms, at least one functional group or a pharmaceutically acceptable salt.