An arteriovenous fistula in the human being offers an excellent opportunity to study the mechanics of the circulation under normal and abnormal conditions in the same person. It is one of the few diseases of the cardiovascular system that can be altered completely and permanently by an operative procedure. A physiologic study of the circulation in patients with an arteriovenous fistula is of interest, not only because of the phenomenal changes that sometimes occur, but because it affords an opportunity for a more complete understanding of other diseases of the cardiovascular mechanism. These observations were undertaken in an attempt to determine the physiologic effect produced on the circulation by a large fistulous communication of six years' duration between the deep femoral artery and vein. The circulation was studied before and after obliteration of the fistula by operation. The secondary effects of this long-standing communication were quite marked; the heart was