The mechanisms of control of glucagon secretion are largely unknown. In particular, the paracrine role of somatostatin is unclear. We studied its role in the control of glucagon secretion by glucose and KATP channel blockers, using perifused islets and the in situ perfused pancreas. The involvement of somatostatin was evaluated by comparing glucagon release of control tissue or tissue without paracrine influence of somatostatin (pertussis toxin-treated islets, or islets or pancreas from Sst -/- mice). We show that removal of the paracrine influence of somatostatin suppresses the ability of KATP channel blockers or KATP channel ablation to inhibit glucagon release, suggesting that, in control islets, the glucagonostatic effect of KATP channel blockers/ablation is fully mediated by somatostatin. By contrast, the glucagonostatic effect of glucose in control islets is mainly independent of somatostatin for low glucose concentrations (0-7 mmol/l) but starts to involve somatostatin for high concentrations of the sugar (15-30 mmol/l). This demonstrates that the glucagonostatic effect of glucose only partially depends on somatostatin. RT-qPCR and pharmacological experiments indicate that the glucagonostatic effect of somatostatin is mediated by two types of somatostatin receptors, SSTR2 and SSTR3. These results suggest that alterations of the paracrine influence of somatostatin will affect glucagon release. 2018 by the American Diabetes Association.