1. A marked cutaneous vasodilatation has been shown to occur in healthy subjects, but not in insulin-dependent diabetic patients, in response to hypoglycaemia induced by a rapid intravenous bolus injection of insulin. 2. In the present study cutaneous blood flow in response to a gradual decline in blood glucose concentration was investigated in eight young adult diabetic patients and in eight age- and sex-matched control subjects. After a hyperinsulinaemic euglycaemic clamp for 40 min, hypoglycaemia was induced (plasma glucose concentration 2 mmol/l) by a standardized stepwise reduction in the intravenous glucose infusion 3. Blood flow was measured by using a laser Doppler sensor and a cutaneous O2 electrode placed on the medial aspect of the forearm, and a laser Doppler sensor placed on the forehead. 4. No significant change in cutaneous blood flow occurred during euglycaemic hyperinsulinaemia. 5. In control subjects a marked increase in blood flow during hypoglycaemia was observed in the forearm by both methods. No corresponding change was observed in the forehead. 6. In the diabetic patients the increase in cutaneous blood flow was absent in both the forearm and forehead. 7. It is concluded that hypoglycaemia, but not hyperinsulinaemia, is associated with a regional cutaneous vasodilatation in healthy control subjects. This cutaneous vasodilatation is absent in diabetic patients.