The creatinine clearance and urinary pH changes were studied in 12 male dogs in whom the ureters were unilaterally obstructed for periods ranging from 2 days to 27 weeks. The obstruction was then released and the recovery was studied. Using the contralateral kidney as control, significant drop in the creatinine clearance was observed to be consistently associated with failure to produce acidic urine. The obstructed kidneys with urinary pH above 7.00 had significant reduction in creatinine clearance when compared with obstructed kidneys with urinary pH 7.00 or less (p less than 0.005). The marked drop in the creatinine clearance and rise in urinary pH was observed to occur in kidneys obstructed for more than 3 weeks. With obstruction for shorter periods, the ability to produce acidic urine is preserved and the recovery potential is good.