The invention is directed to somatostatin analogs which are receptor antagonists of the somatostatin receptor, including receptor-selective antagonists, especially sst2-selective antagonists. Related compounds and compositions are included, including antagonists complexed with or conjugated to radioactive nuclides. The antagonists of the invention are useful in diagnosing and treating neoplastic and non-neoplastic mammalian diseases; such methods, and kits, are encompassed.