Summary.Renal blood flow has been estimated by measuring the venous return from both kidneys simultaneously in acute experiments on rabbits.Control experiments showed no significant difference between the two normal kidneys.From 1 to 17 weeks after ligation of one ureter the blood flow was reduced to about 40 percent on the obstructed side while there was a corresponding increase on the other side. The alterations developed rapidly during the first week and were fairly constant after the fourth week of obstruction.Kidney weight increased markedly on the obstructed sideduring the first three weeks while there was a slight increase only on the other side.Pelvic fluid pressure increased during the first two weeks whereafter a slow fall was observed. The quantity of fluid. on the other hand, increased up to the sixth week.