Continuous, low dose, insulin infusion in conscious dogs produced moderate hypoglycemia but only a transient fall in glucose production that rose towards preinfusion levels 20 to 30 min before any detectable increase in plasma counterregulatory hormones. Addition of epinephrine or glucagon to the insulin infusion prevented the fall in glucose production throughout the experiment but only partially diminished the hypoglycemic response. When hypoglycemia was prevented by a variable glucose infusion, neither epinephrine nor glucagon was able to counteract the suppressive effect of insulin on glucose output. These findings suggest that a fall in blood glucose per se may reverse insulin-induced inhibition of glucose production independent of a rise in counterregulatory hormones and that the insulin antagonist effect of counterregulatory hormones is modulated, at least in part, by blood glucose concentration.