Muscle fatigue is the most disabling symptom in primary fibromyalgia (PF), which in addition is characterized by generalised pain and muscle stiffness. In order to assess whether the fatigue is of central and/or peripheral origin, skeletal muscle function was studied by measuring maximum voluntary hand grip strength, and by measuring various contraction characteristics in the adductor pollicis muscle after electrical stimulation of the ulnar nerve. The PF-patients were also studied after a regional sympathetic blockade of the forearm with guanethidine. A lower hand grip strength was found in the PF-patients compared to the controls, before as well as during the sympathetic blockade. The developed force, measured during electrical stimulation, did not differ between patients and controls. A lower muscle relaxation rate was found in the PF-patients. The relaxation rate increased in the PF-patients during the sympathetic blockade. The results indicate both a central and a peripheral cause of muscle dysfunction. Activity in the muscle sympathetic system may be one link in the chain of events that leads to muscular symptoms in PF.