Abstract: The invention relates to gonadoliberin derivatives of the general formula (I)Glp--His--Trp--Ser--Tyr--X.sub.1 --X.sub.2 --X.sub.3 --Pro--X.sub.4whereinX.sub.1 is a glycyl group or a D-isomer of any natural or synthetic amino acid group,X.sub.2 represents an L-amino acid group having 1 to 4 carbon atoms in the side chain, L-phenyl-alanyl or L-tryptophyl group,X.sub.3 represents an L-amino acid group having a C.sub.1-4 alkyl or C.sub.2-4 alkanoyl-amide side chain, andX.sub.4 is a glycine amide or a C.sub.1-4 alkyl amide group,with the proviso that if X.sub.1 stands for a group other than glycyl and X.sub.2 is a tryptophyl group then X.sub.3 may not be leucyl, and the addition salts formed with therapeutically useable acids and complexes thereof. Furthermore the invention relates to a process for preparing these compounds.The new gonadoliberin derivatives can be effectively used for the reproduction process of fish, birds and mammals.