Objective: To examine the effects of intravenous insulin and acute hypoglycemia on arterial wall stiffness and central hemodynamic responses in adults with and without type 1 diabetes. Research Design and Methods: Intravenous insulin was administered to thirty young male volunteers (10 non-diabetic (Group 1); 10 with type 1 diabetes <5 years duration (Group 2); 10 with type 1 diabetes >15 years duration (Group 3)) to provoke an acute autonomic reaction (R) to hypoglycemia. Heart rate, peripheral blood pressure (BP) and pulse wave analysis (radial artery) were monitored. Augmentation index (AIx), a measure of arterial wall stiffness, and central BP were recorded. Results: At baseline no significant differences were observed between Groups 1 and 2 in either AIx or in central BP. In Group 3 both measures were significantly higher. All groups exhibited similar responses to intravenous infusion of insulin and to hypoglycemia: AIx fell progressively from baseline to R, peripheral systolic BP increased, while central systolic BP decreased Conclusion: When compared to age- and sex-matched non-diabetic controls, people who had type 1 diabetes of long duration had increased stiffness of vessel walls. The opposing responses in peripheral and central BP during hypoglycemia may be related to the reduction in AIx, which causes diminished amplification of the systolic pressure wave. Changes in AIx are probably mediated by a direct action of insulin on arterial endothelium, or changes in heart rate. These functional changes may contribute to the increased cardiovascular morbidity that is associated with type 1 diabetes of long duration.