OBJECTIVE To determine whether reduced hormonal, symptomatic, and/or cognitive responses to hypoglycemia are caused by an increase in the plasma glucose concentration required to stimulate these counterregulatory parameters after antecedent hypoglycemia. METHODS We studied nine healthy volunteers during stepped hypoglycemia clamps (plasma glucose targets from 80 to 50 mg/dl in 10 mg/dl steps) on two separate days. The study was preceded either by a 2-h period of hypoglycemia (plasma glucose 58 +/- 2 mg/dl) or a 2-h period of euglycemia (plasma glucose 94 +/- 2 mg/dl) for 90 min. RESULTS The plasma glucose that triggered secretion of plasma norepinephrine (NE) was lower after antecedent hypoglycemia (control = 74 +/- 2 and experimental = 67 +/- 2 mg/dl, respectively, P < 0.005). In contrast, a relatively higher plasma glucose stimulated secretion of other counterregulatory hormones after antecedent hypoglycemia: growth hormone (GH) (65 +/- 2 to 72 +/- 2 mg/dl, P < 0.01); glucagon (63 +/- 2 to 70 +/- 2 mg/dl, P < 0.01); and epinephrine (EPI) (68 +/- 2 to 76 +/- 2 mg/dl, P < 0.01) when comparing control days with experimental days. Hypoglycemic symptoms were first observed at a plasma glucose plateau of 59 +/- 2 mg/dl. Motor function reflected by Digit Symbol Substitution deteriorated equally whether there had been antecedent hypoglycemia or euglycemia. Logical (immediate) memory deteriorated in the control study at a plasma glucose of 54 +/- 2 mg/dl but remained unchanged at equivalent hypoglycemia in the experimental study (P < 0.03). CONCLUSIONS Our conclusions are as follows: 1) symptoms of moderate hypoglycemia occur at plasma glucose levels averaging approximately 5-15 mg/dl lower than the plasma glucose concentrations required to trigger counterregulatory hormone release; 2) after acute antecedent hypoglycemia, glucagon, EPI, and GH secretion occur at higher plasma glucose concentrations and NE is released at lower plasma glucose concentrations; and 3) there may be CNS adaptation to prior hypoglycemia reflected in preservation of logical memory function at plasma glucose levels of approximately 50 mg/dl. These findings suggest that thresholds for hormone secretion and for changes in cognitive function can be altered very acutely by foregoing hypoglycemia in healthy humans.