The effect of somatostatin on glucoseinduced secretion of insulin and gastric inhibitory polypeptide (GIP) in rats was examined. Anesthetized male rats were catheterized via the right carotid artery before gastric infusion of 2 ml of 40% glucose through a syringe pump over 3 min. Ten micrograms somatostatin dissolved in 0.5 ml saline was injected intraperitoneally 10 min before oral glucose administration. Rats receiving 0.5 ml saline were used as controls. Blood samples were collected at -10, 0, 10, 20, 30, 45, 60, and 90 min after glucose infusion. Concentrations of GIP and insulin in rat plasma samples were measured by radioimmunoassay. The level of plasma glucose was determined by YSI glucose analyzer. The results revealed that the rise of plasma insulin and GIP after the glucose load was abolished by a preinjection of somatostatin. It is concluded that somatostatin at 10 micrograms inhibits the effects of oral glucose in stimulating the secretion of GIP from the gut, and insulin from the pancreas in normal fasted rats.