The effects of insulin on vasoconstrictor response were studied in diabetics without autonomic neuropathy. Vasoconstrictor response to phenylephrine was measured before and after insulin injection in 10 diabetics. Mean blood pressure, forearm blood flow, and forearm vascular resistance remained unchanged after insulin administration. But the increase in forearm vascular resistance to phenylephrine decreased significantly and heart rate increased significantly after insulin injection. In 9 diabetics, vasoconstrictor response to tilting was measured before and after insulin injection. Mean blood pressure, cardiac index, and the total peripheral resistance index remained unchanged after insulin administration. However, the increase in the total peripheral resistance index to tilting decreased significantly and heart rate increased significantly after insulin injection. Furthermore, a significant fall in mean blood pressure following tilting was observed only after insulin administration. Although blood glucose levels decreased significantly, no hypoglycemic symptoms occurred. The present study suggests that insulin inhibits vasoconstrictor responses to both alpha agonist and tilting in diabetics.