a 1 - cyclo - a 5 - y 1 the present invention is directed to a somatostatin antagonist according to formula , wherein a 1 is an optionally substituted aromatic ax - amino acid ; a 2 is an optionally substituted aromatic α - amino acid ; a 3 is dab , dap , lys or orn ; a 4 is β - hydroxyvaline , ser , hser , or thr ; a 5 is an optionally substituted d - or l - aromatic - amino acid ; and y 1 is oh , nh 2 or nhr 1 , where r 1 is 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 , no 2 , oh , cn , alkyl , alkenyl , alkynyl , alkoxy , bzl , o - bzl , and nr 9 r 10 , where r 9 ad r 10 each is independently h , o , or alkyl ; and wherein the amine nitrogen of each of amide peptide bond and the amino group of a 1 of formula is optionally substituted with a methyl group , provided that there is at least one said methyl group ; or a pharmaceutically acceptable salt thereof , and to uses thereof .