A metastin derivative that is PEG5K (CS) -CO-Acp-Tyr-Asn-Trp-Asn-Ser-Phe-Gly-Leu-Arg-Phe-NH2, PEG2K (CS) -CO-Acp-Tyr-Asn- Trp-Asn-Ser-Phe-Gly-Leu-Arg-Phe-NH2, PEG20K (CS) -CO-Acp-Tyr-Asn-Trp-Asn-Ser-Phe-Gly-Leu-Arg-Phe-NH2, PEG40K (AL) -CH2-Acp-Tyr-Asn-10 Trp-Asn-Ser-Phe-Gly-Leu-Arg-Phe-NH2, PEG20K (HS) -CO-Acp-Tyr-Asn-Trp-Asn-Ser- Phe-Gly-Leu-Arg-Phe-NH2, PEG40K (HS) -CO-Acp-Tyr-Asn-Trp-Asn-Ser-Phe-Gly-Leu-Arg-Phe-NH2, PEG30K (AL) -CH2- Acp-Tyr-Asn-Trp-Asn-Ser-Phe-Gly-Leu-Arg-Phe-NH2, PEG20K (CS) -CO-Acp-D-Tyr-D-Trp-Asn-Thr-Phe-AzaGly-Leu -Arg (Me) -Trp-NH2, PEG20K (CS) -CO-Acp-D-Tyr-Hyp-Asn-Thr-Phe-AzaGly-Leu-Arg (Me) -Trp-NH2, PEG20K (CS) -CO -Acp-D-Tyr-Glu-Asn-Thr-Phe (3F) -AzaGly-Leu-Arg (Me) -Trp-NH2, or PEG20K (CS) -CO-Acp-D-Tyr-Hyp-Asn-Thr -Phe (4F) -AzaGly-Leu-Arg (Me) -Trp-NH2; or a salt thereof (in which the numbers and K mean kDa, which is the molecular weight of PEG, (CS) represents -CO (CH2) 2-, (AL) represents - (CH2) 2- and (HS) represents - (CH2) 5-.).Un derivado de metastina que es PEG5K(CS)-CO-Acp-Tyr-Asn-Trp-Asn-Ser-Phe-Gly-Leu-Arg-Phe-NH2, PEG2K(CS)-CO-Acp-Tyr-Asn-Trp-Asn-Ser-Phe-Gly-Leu-Arg-Phe-NH2, PEG20K(CS)-CO-Acp-Tyr-Asn-Trp-Asn-Ser-Phe-Gly-Leu-Arg-Phe-NH2, PEG40K(AL)-CH2-Acp-Tyr-Asn-10 Trp-Asn-Ser-Phe-Gly-Leu-Arg-Phe-NH2, PEG20K(HS)-CO-Acp-Tyr-Asn-Trp-Asn-Ser-Phe-Gly-Leu-Arg-Phe-NH2, PEG40K(HS)-CO-Acp-Tyr-Asn-Trp-Asn-Ser-Phe-Gly-Leu-Arg-Phe-NH2, PEG30K(AL)-CH2-Acp-Tyr-Asn-Trp-Asn-Ser-Phe-Gly-Leu-Arg-Phe-NH2, PEG20K(CS)-CO-Acp-D-Tyr-D-Trp-Asn-Thr-Phe-AzaGly-Leu-Arg(Me)-Trp-NH2, PEG20K(CS)-CO-Acp-D-Tyr-Hyp-Asn-Thr-Phe-AzaGly-Leu-Arg(Me)-Trp-NH2, PEG20K(CS)-CO-Acp-D-Tyr-Glu-Asn-Thr-Phe(3F)-AzaGly-Leu-Arg(Me)-Trp-NH2, o PEG20K(CS)-CO-Acp-D-Tyr-Hyp-Asn-Thr-Phe(4F)-AzaGly-Leu-Arg(Me)-Trp-NH2; o una sal del mismo (en las que los números y K significan kDa, que es el peso molecular de PEG, (CS) representa -CO(CH2)2-, (AL) representa -(CH2)2- y (HS) representa -(CH2)5-.).