Glucagon and epinephrine are the most important short term glucose counterregulatory hormones. The epinephrine response in patients with insulin-dependent diabetes mellitus is related to the level of glycemic control, but little is known about the factors influencing counterregulation in normal subjects. We, therefore, conducted hyperinsulinemic glucose clamp studies to examine the counterregulatory response to recurrent and prolonged mild hypoglycemia in normal women. Blood glucose was clamped for 20 min at 3.5 mmol/L. Thereafter, the subjects had their blood glucose maintained at 2.8 mmol/L for 90 min and on another occasion lowered to 2.8 mmol/L and raised to 3.5 mmol/L twice during the 90-min period. Continuous hypoglycemia produced augmented plasma glucagon, cortisol, and pancreatic polypeptide responses (all P less than 0.05) compared to these responses to recurrent hypoglycemia. Plasma GH increased, but the magnitude of the response was not altered by the duration of hypoglycemia. During the recurrent hypoglycemia study plasma epinephrine levels rose and fell in parallel with the fluctuations in blood glucose. The mean peak increase was similar [1.37 +/- 0.25 (+/- SE) nmol/L] to that during the continuous study (1.76 +/- 0.23 nmol/L). There was no change in plasma glucagon levels in response to hypoglycemia of less than 15-min duration. We conclude that 1) the duration of hypoglycemia influences the counterregulatory response, and 2) epinephrine release is under precise control and responds rapidly to fluctuations in blood glucose.