A group of new peptide ligands displaying high selectivity for binding to somatostatin receptor subtypes 2, 3 or 5 have been used to characterize somatostatin receptor involvement in the inhibition of glucagon secretion in rats. It was found that NC-8-12 and DC-25-100, which have high affinity for SSTR2 and much less affinity for the type 5 receptor, were by far the most potent inhibitors of glucagon secretion with EC50s of 48 and 18 nmole, respectively, relative to somatostatin itself (EC50 131 nmole). These two analogs were actually much less potent than somatostatin in inhibiting glucose-stimulated insulin release. In contrast, DC-23-99 (a type 5 receptor selective analog), which was previously found to be a more potent inhibitor of insulin secretion than somatostatin, had considerably less potent (EC50 410 nmole) effects on glucagon release. The SSTR3-specific ligands, DC-25-12 and DC-25-20, were not effective at the doses tested. The differing spectra of activities of these analogs suggest that inhibition of insulin and glucagon secretion in rats is mediated by entirely different somatostatin receptor populations.