PRIOR to the introduction of the reactive hyperemia test by Matas,1 there was no method of assaying before operation whether surgical occlusion of the great arteries would be followed by relatively adequate circulation in the extremity or whether some ischemic disaster would result. Indeed such precautions are still sometimes omitted, if one is to judge by the lack of mention of any tests for adequacy of collateral circulation in a number of reports and monographs dealing with the surgery of aneurysms and arteriovenous fistulas. It is our conviction that in general the age of the patient, the duration of the aneurysm or fistula and the general appearance of the affected extremity offer nothing conclusive in regard to the safety with which ligation of the involved vessel may be carried out. It is obvious that the ideal method of treating such lesions is to restore the continuity of blood flow