Abstract: The present invention is directed to a somatostatin antagonist according to formula (I): A1-cyclo {D-Cys-A2-D-Trp-A3-A4-Cys}-A5Y1, wherein A1 is an optionally substituted aromatic a-amino acid; A2 is an optionally substituted aromatic ?-amino acid; A3 is Dab, Dap, Lys or Om; A4 is ?-Hydroxyvaline, Ser, Hser, or Thr; A5 is an optionally substituted D- or L-aromatic amino acid; and Y1 is OH, NH2 or NHR1? where RI is (C1-6)alkyl; wherein each said optionally substituted aromatic amino acid is optionally substituted with one or more substituents each independently selected from the group consisting of halogen, NO2, OH, CN, (C1-6) alkyl, (C2-6) alkenyl, (C2-6) alkynyl, (C1-6)alkoxy, Bzl, O-Bzl, and NR9R10? wherein R9 ad R10 each is independently H, O, or (C1-6) alkyl; and wherein the amine nitrogen of each of amide peptide bond and the amino group of A1 of formula (I) is optionally substituted with a methyl group, provided that there is at least one said methyl group; or a pharmaceutically acceptable salt thereof,