Exendin-4 is a potent and long-acting agonist of the glucagon- like peptide-1 (GLP-1) receptor. GLP-1 is an insulinotropic gut peptide and is being evaluated for the regulation of plasma glucose in type 2 diabetes. The purpose of the present study was to ascertain whether exendin-4 is insulinotropic and whether it has long-lived biological effects in nondiabetic and type 2 diabetic subjects. Because incretins are glucose depen- dent with respect to their insulin-releasing capacity, we used the hyperglycemic glucose clamp technique to begin to ad- dress these issues in two separate protocols. In one protocol, we infused exendin-4 (0.15 pmolkg1min1) in seven nondi- abetic and seven type 2 diabetic subjects during the second hour of a 5-h hyperglycemic clamp in which fasting plasma glucose was raised by 5.4 mmol/liter. The second protocol was identical to the first except that plasma glucose was allowed to fall to the fasting levels during the fourth hour and again raised by 5.4 mmol/liter during the fifth hour in four nondi- abetic and four diabetic subjects. With the initiation of exendin-4 infusion at 60 min, plasma insulin response was potentiated 4- to 5-fold in both groups. Despite termination of exendin-4 at the end of the second hour, the insulin levels remained elevated for several hours and hyperglycemia was maintained. All volunteers ate a meal 5.5 h after inducing hyperglycemia. Postprandial plasma glucose, insulin, and GLP-1 did not rise in any subject, possibly because of delayed gastric emptying by exendin-4 even though its infusion had been terminated 4 h previously. We concluded that exendin-4 is a potent and long-lasting insulinotropic agent in nondia- betic and diabetic subjects. (J Clin Endocrinol Metab 87: 1282-1290, 2002)