After stable mild or severe partial unilateral ureteral obstruction in rats, the ratio of renal dry weight to whole body weight changes in two phases. In a "destructive phase" of two to six weeks, slight weight reduction occurs in the kidneys with mild obstruction and pronounced weight reduction occurs in those with severe obstruction. Then, in a "steady-state phase", there is no further weight reduction in kidneys with either mild or severe obstruction. Relief of ureteral obstruction during the steady-state phase does not result in an increase in renal dry weight. We conclude that the development of hydronephrotic atrophy after stable mild or severe partial unilateral ureteral obstruction can not be influenced by relief of obstruction in the steady-state phase.