These experiments were undertaken to demonstrate the participation of the cholinergic system in the reflex vasodilatation in the gracilis muscle of the dog evoked by rapid i.v. injection of norepinephrine. The analysis of the integrated areas of vasodilatation curves after pretreatment with atropine showed a significant decrease of the reflex vasodilatation suggesting the participation of the cholinergic system to the mechanism underlying the phenomenon. The residue vasodilatation was completely abolished by the subsequent i.a. administration of phentolamine. On the other hand, guanethidine too could, also without pretreatment with atropine, nullify the reflex vasodilatation. This effect, therefore, does not permit the ruling out of the existence of a cholinergic component in the reflex vasodilatation since the actions of these drugs are not yet completely known.