The influence of maximum exercise has been studied in 10 patients with primary fibromyalgia syndrome (PFS) and 10 healthy sedentary control persons. The exercise consisted of a bicycle ergometertest and a steptest, both till exhaustion. In both tests, the mean maximum workload of the PFS patients was lower than that of the controls. Significantly lower values of serum creatinekinase, myoglobin, cortisol, epinephrine and norepinephrine were found in PFS patients. A striking finding was a lower heart rate in PFS patients compared to the controls under the same workload. The lower (nor)epinephrine concentration together with the lower heart rate suggests a disturbance of the sympathetic activity in PFS patients. The preliminary conclusion is that there is a disturbed reactivity of the sympathetic system as well as of the HPA axis in PFS.