Somatostatin receptors (SSTRs) are activated by somatostatin secreted from nerve and endocrine cells. SSTRs are expressed in a tissue-specific manner and involved in the regulation of secretion of insulin, glucagon and growth hormone as well as cell growth induced by neuronal excitation in both the central and peripheral nervous systems. Aberrant expression of somatostatin receptors is known in a large number of human tumors. SSTR1 has been reported at highest levels in the jejunum and stomach. In addition, this receptor is expressed in adrenal, brain, liver, lung, eye, and pancreas. Selective activation of SSTR1 inhibits hormone secretion and cell viability in GH-secreting and PRL-secreting adenomas in vitro.