After validation of an isotope delutions method for quantitaling forearm norepinephrine release and thus sympathetic neural activity in response to standing and hypoglycemia, we will test the hypothesis that glycemic thresholds for sympathetic neural responses to hypoglycemia, like those for adrenomedullary and parasympathetic neural responses are elevated (lower plasma glucose concentrations required) following recent antecedent hypoglycemia in humans. Using the stepped hypoglycemic clamp technique to assess responses, healthy subjects will be studied on two occasions, once following clamped hypoglycemia (approximately 2.5 mmol/Lx2) and once following otherwise identical clamped euglycemia (approximately 5.6 mmol/Lx2h) the previous afternoon in random sequence.