OBJECTIVE: Antecedent hypoglycemia can blunt neuroendocrine and autonomic nervous system responses to next-day exercise in type 1 diabetes. The aim of this study was to determine whether antecedent increase of plasma cortisol is a mechanism responsible for this finding. RESEARCH DESIGN AND METHODS: For this study, 22 type 1 diabetic subjects (11 men and 11 women, age 27 +/- 2 years, BMI 24 +/- 1 kg/m(2), A1C 7.9 +/- 0.2%) underwent four separate randomized 2-day protocols, with overnight normalization of blood glucose. Day 1 consisted of morning and afternoon 2-h hyperinsulinemic- (9 pmol x kg(-1) x min(-1)) euglycemic clamps (5.1 mmol/l), hypoglycemic clamps (2.9 mmol/l), or euglycemic clamps with a physiologic low-dose intravenous infusion of cortisol to reproduce levels found during hypoglycemia or a high-dose infusion, which resulted in further twofold greater elevations of plasma cortisol. Day 2 consisted of 90-min euglycemic cycling exercise at 50% Vo(2max). RESULTS: During exercise, glucose levels were equivalently clamped at 5.1 +/- 0.1 mmol/l and insulin was allowed to fall to similar levels. Glucagon, growth hormone, epinephrine, norepinephrine, and pancreatic polypeptide responses during day 2 exercise were significantly blunted following antecedent hypoglycemia, low- and high-dose cortisol, compared with antecedent euglycemia. Endogenous glucose production and lipolysis were also significantly reduced following day 1 low- and high-dose cortisol. CONCLUSIONS: Antecedent physiologic increases in cortisol (equivalent to levels occurring during hypoglycemia) resulted in blunted neuroendocrine, autonomic nervous system, and metabolic counterregulatory responses during subsequent exercise in subjects with type 1 diabetes. These data suggest that prior elevations of cortisol may play a role in the development of exercise-related counterregulatory failure in those with type 1 diabetes.