Intraneural stimulation (i.n.s.) was made in the superficial peroneal nerve at the ankle in seventeen healthy subjects. The effect on skin blood flow was monitored by laser-doppler flowmeters and photo-electrical pulse plethysmographs inside and outside the innervation zone of the stimulated nerve fascicle. I.n.s. was applied before and after proximal local anaesthesia of the stimulated nerve. 2. Painful i.n.s. (stimulation strength 0.3-4 V) induced skin vasodilatation with the following characteristics: (a) it occurred on the dorsal side of both feet, (b) the blood flow increase on the opposite foot was blocked by local anaesthesia of the nerve supplying the skin area under study, (c) the blood flow increase on the stimulated foot was abolished by proximal local anaesthesia of the stimulated nerve. The findings show that the vasodilatation was due to activation of a reflex pathway. 3. The reflex vasodilatation was bigger in the stimulated than in the opposite foot. At the same time there were signs of skin vasoconstriction in the fingers. The reflex vasodilatation in the foot was enhanced by body cooling. It was unaffected by atropine or propranolol. 4. After local anaesthesia of the nerve proximal to the stimulation site, i.n.s. with 2-6 times increased stimulation strength produced skin vasodilatation restricted to the innervation zone of the stimulated nerve fascicle. This response had greater amplitude and longer duration than the reflex vasodilatation. 5. Intravenously administered atropine and propranolol did not affect the local dilatation to i.n.s. but after chronic treatment of the skin with capsaicin (1% in ethanol), i.n.s. after a proximal nerve block induced skin vasoconstriction. In five of seven experiments subcutaneous injection of terbutaline (0.25 mg) in the innervation zone abolished the local dilatation. 6. It is suggested that (a) the reflex vasodilatation is of sympathetic nature and is induced by stimulation of thin (A delta?) afferent fibres, (b) the local vasodilatation is due to centrifugally conducted impulses in (afferent?) non-myelinated fibres.
