A frequent, though by no means invariable, phenomenon observed clinically in arteriovenous fistulas has been the increase in the size of the heart, associated with an increase in the size of the artery and vein proximal to the fistula. Since William Hunter's first description of a proximal dilatation of the artery in a case of arteriovenous aneurysm, this phenomenon has puzzled physicians in their efforts to develop an adequate explanation for its occurrence in certain instances and its absence in others. Most writers, unwilling to accept a purely mechanical explanation of the changes, attribute them to a vital factor comparable to that which causes the vessels of a gravid uterus or of a growing tumor to dilate. Others have attributed the proximal dilatation to a lessened arterial tension, a disuse atrophy. Few explanations of the cardiac enlargement are available. Lewis and Drury1 explain it on the basis of a