OBJECTIVE To evaluate the effect of angiotensin receptor blocker (losartan) on renal function during and after relief of partial unilateral ureteral obstruction (PUO). METHODS A total of 32 male mongrel dogs were classified into 3 groups: sham (8), control (12; left PUO + no medications), and study (12; left PUO + losartan). Dogs of the study and control groups were subjected to 4 weeks of PUO. After that, they were reopened and subjected to Lich-Grigoir ureterovesical reimplantation and then were killed by the end of 32 weeks after relief of obstruction after being evaluated at basal condition; fourth week of obstruction; and at 4, 8, and 32 weeks after relief of obstruction by measurement of selective creatinine clearance (CCr), selective renographic clearance (RC), and renal resistive index. Sham group underwent sham surgery at 4 and 32 weeks and evaluated as the other 2 groups. RESULTS Sham surgery showed no significant effect on any of the evaluated parameters. Compared with the control, losartan saved reduction in CCr by 11% and RC by 20% of the basal value by the end of the fourth week of obstruction, respectively. Moreover, compared with the control, losartan enhanced regain of CCr by 26% and RC by 26% also of the basal value at 32 weeks after relief of fourth week obstruction, respectively. In addition, the increase in renal resistive index was significantly less in the losartan group. CONCLUSIONS Losartan decreases the deterioration of renal function in PUO and enhances recoverability of renal function after relief of obstruction.