Changes in the sympathetic nerve system have been suggested as the pathophysiological mechanism underlying vibration white finger (VWF). The aim of the present study was to investigate if experimental support for such a mechanism could be found in VWF. Drugs with a known effect on sympathetic alpha receptors were administered into the finger skin by iontophoresis and their effects on blood flow in the same area evaluated using a laser Doppler technique. The effects of noradrenaline (stimulating alpha-1 and alpha-2 receptors), phenylephrine (an alpha-1 stimulator), and B-HT 933 (an alpha-2 stimulator) were studied in 12 patients with vibration white finger and 12 healthy controls. The reactions to noradrenaline and B-HT 933 were similar in both patients and controls, but the reaction of the patients to phenylephrine was significantly weaker than the controls. In additional experiments in six patients and six controls concentration effect curves to phenylephrine were derived. The curves for the patients were shifted to the right--that is, they reacted less strongly than the controls at all doses of the drug which induced an appreciable vasoconstriction. The results of this study are compatible with the hypothesis that the alpha-1 receptor mediated responses are weakened in VWF. The predominance of alpha-2 receptors in the digital arteries has, on the basis of animal experiments, been suggested as a possible mechanism for Raynaud's phenomenon.