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Purpose: Our obstructive uropathy model in fetal lambs showed that renal cystic changes appeared 3 weeks after obstruction. In this study, the authors investigated the changes resulting from complete urinary tract obstruction in the first 7 days after obstruction., Methods: An obstructive uropathy was created in fetal lambs at 60 days' gestation by ligating the urethra and urachus. They were delivered 48 hours, 3 days, 5 days, and 7 days later by cesarian section. The kidneys were removed and processed for histologic examination., Results: Eighteen fetuses were operated on and 15 (4 at 48 hours, 4 at 3 days, 2 at 5 days, and 5 at 7 days; 83%) survived. Macroscopically, bladder dilatation and slightly dilated ureters were identified from 48 hours. Microscopically, dilatation of proximal tubules started from 48 hours after obstruction and increased by 7 days. Glomerular cysts in the nephrogenic zone also were identified from 48 hours. Dysplastic changes were not found., Conclusions: The first areas in the developing kidney that suffer damage after obstructive uropathy are the proximal tubule and the nephrogenic zone. This change started 48 hours after obstruction. Shunting procedures need to be performed considerably earlier than previously thought.