Attacks of cluster headache provoked by the administration of nitroglycerin (1 mg sublingually) or histamine (0.01 mg/kg subcutaneously) were studied. The constant latency time in individual patients during nitroglycerin and histamine provocation suggested that the same mechanism is involved in both methods of headache induction. Repeated nitroglycerin provocation revealed its tendency to lengthen duration of the refractory period at the end of the bout, when the attacks were less frequent. The "cross studies" on spontaneous and nitroglycerin provoked attacks indicated that the underlying mechanism of both is at least partially the same.