Effects of somatostatin on fasting and arginine-or tolbutamide-stimulated insulin release were studied in four patients with insulinoma. Somatostatin (bolus or bolus + infusion) reduced fasting insulin values in all patients; insulin response to tolbutamide was partially reduced in two patients; somatostatin bolus impaired the insulin response to arginine. Fasting glucagon levels and glucagon response to arginine were also reduced by somatostatin. These results indicate the potential usefulness of somatostatin in the diagnosis of insulinoma even if its effect on insulin is only partial.