In diabetes, loss of awareness of and a defective hormonal response to hypoglycaemia have been associated with long disease duration, improved glycaemic control and possibly a change in insulin species. In contrast it is assumed that normal subjects always have symptoms when their blood glucose is low. We have tested this in 7 normal subjects at 3 levels of blood glucose (4.5, 3.5 and 3.0 mmol/l) using a hyperinsulinaemic glucose clamp with a euglycaemic (4.5 mmol/l) clamp as a control. After 60 min at a blood glucose of 3.5 mmol/l adrenaline and glucagon increased slightly but significantly, whereas cortisol, growth hormone and pancreatic polypeptide were unchanged. As soon as glucose was lowered to 3.0 mmol/l adrenaline increased to 1.10 nmol/l and rose further to 1.43 nmol/l after 60 min. Glucagon secretion increased similarly but other counter-regulatory hormones were significantly raised only after 60 min at 3.0 mmol/l. Awareness of hypoglycaemia (symptom score) increased after 40 min at a blood glucose of 3.0 mmol/l but after 60 min decreased to baseline levels with loss of awareness in 5 subjects. Reaction time improved in parallel with the change in symptom score. Thus, despite high levels of adrenaline, normal subjects lose awareness during sustained mild hypoglycaemia. Improved reaction time may reflect cerebral adaptation.