1. The response of hand blood flow to acute insulin-induced hypoglycaemia was measured in normal subjects in whom vasomotor tone had been released physiologically by body heating, anatomically by unilateral nerve block and pharmacologically by unilateral intra-arterial infusion of bretylium tosylate or phenoxybenzamine. Five patients who had undergone unilateral cervical sympathectomy were also studied.
2. Under these conditions the usual increase in hand blood flow associated with acute hypoglycaemia was converted into a decrease except during complete unilateral α-adrenergic blockade when hypoglycaemia produced no change in hand blood flow.
3. The possible mechanisms responsible for the vasodilatation associated with hypoglycaemia are discussed.
4. The results suggest that release of vasoconstrictor tone, rather than activation of specific vasodilator mechanisms (neurogenic or humoral), is the principal means whereby acute hypoglycaemia produces increased blood flow in the hand.