Partial ureteric obstruction was produced by insertion of an obstructing stent into the left ureter of 21 mongrel dogs and the duration of obstruction was varied as follows: Group A (n = 7) for 60 days; Group B (n = 7) for 28 days; Group C (n = 7) for 14 days. Intrapelvic pressure monitoring confirmed that obstruction had been produced by stent insertion and relieved by reimplantation of the left ureter following the designated obstruction period. The extent of recovery of renal function was assessed by creatinine clearance, with group C maintaining normal function, group B recovering 31% and group A recovering 8% of control function. Methyl-methacrylate extrusion casts of the renal microvasculature were made in group B and C kidneys and studied by scanning electron microscopy. During obstruction there was evidence of arteriolar constriction. These changes provide a morphological basis for the renal functional alterations and support the concept that post-obstructive renal failure is a vascular injury.