The effect on the kidney of the ligation of one branch of the renal artery has been the object of much investigation, concerned chiefly with the question of an arterial collateral circulation as shown by the pathologic changes, particularly relative to regeneration in the infarcted area,1 and rarely with the question of functional activity and restoration.2 It is the purpose of this paper to present an experimental study in the rabbit of the effect of this procedure on the rate of development of hydronephrosis. ANATOMIC CONSIDERATIONS The arterial distribution of the kidney is divided almost equally into an anterior and a posterior division which are entirely devoid of anastomoses or intercommunications.3 Each is an independent and an end system. In the majority of animals with one-lobed kidneys (dog, sheep, rat and cat), the posterior division is the larger and supplies a greater area of parenchyma than the