This invention is directed to peptides related to somatostatin and to methods for pharmaceutical treatment of mammals using such peptides. More specifically, the invention relates to shortened receptor-selective somatostatin peptide antagonists and the inclusion of amino acid substitutions and/or additions in such peptides that confer receptor-selectivity thereto, to pharmaceutical compositions containing such peptides, to such peptides complexed with or conjugated to radioactive nuclides, to methods of diagnostic and therapeutic treatment of neoplastic and non-neoplastic mammalian diseases using such peptides, particularly peptides that are coupled to chelators or otherwise labeled, and also to methods for screening for more effective drugs using such peptides.