Abstract.A rapid intravenous glucose load (20 g) was given with a phentolamine infusion during and after elective abdominal surgery. Plasma levels of glucose, free fatty acids, and insulin were measured to investigate the influence of surgical stress on insulin secretion.When Ringer's lactate solution was infused into a control group of subjects during surgery, plasma levels of insulin did not change during and after the surgery while plasma levels of glucose and free fatty acids increased gradually during this period. Similar results were also noted in another control group in whom only Ringer's lactate solution and phentolamine had been infused. This evidence suggests that insulin secretion responding to endogenous hyperglycaemia is suppressed during surgery.In the group which was given the glucose load during infusion of only Ringer's lactate solution, plasma levels of insulin significantly increased soon after the glucose load and then gradually decreased. In another group which was given the glucose load during infusion of Ringer's lactate and phentolamine, plasma levels of insulin also increased significantly after the glucose load and remained elevated during surgery. The maximum increment of plasma insulin after the glucose load in the latter group was significantly higher than that in the former group.From these results it is suggested that suppression of insulin secretion by surgical stress is inhibited by the alpha blocking agent phentolamine.