The effect ofclonidine, a centrally acting drug diminishing the sympathetic activity of peripheral nerves, was examined in a double-blind study of 60 forest workers with vibration-induced white fingers. Clonidine was administered in daily doses of 0.450 mg or 0.100 mg during 30 d. The results were evaluated from a self-administered questionnaire. No significant differences were discerned between the active and placebo groups in respect to either recovery time after an attack or frequency of attacks. Neither did the subjective evaluation of the overall effectiveness of the drug show any differences between the groups. In two subjects clonidine provoked Raynaud's phenomenon, which subsided after the cessation of treatment. The recording of finger pulse plethysmography for two subjects treated with 0.450 mg of clonidine daily revealed comparatively well preserved vascular reflexes, whereas sympathetic mass reflexes such as galvanic skin response were almost absent.