COMPLETE URETERAL OBSTRUCTION initiates a sequence of events which leads ultimately to the complete destruction of its kidney. Although short periods of obstruction produce renal damage,1this has been shown to be almost completely reversible after its release.2This study was undertaken to determine the critical length of time a kidney could withstand complete obstruction. The end point was considered to have been exceeded when the kidney showed no evidence of reparative ability following removal of the obstruction. Materials and Methods Preliminary tubular (phenolsulfonphthalein) and glomerular (mannitol) function studies were obtainedfrom the left kidney in healthy mongrel dogs. The left ureter was then ligated and divided near the ureterovesical junction and implanted in an easily accessible subcutaneous pouch. These were allowed to remain completely obstructed from 1 to 5 weeks. Following removal of the ureteral ligature, the preliminary function studies were obtained after 24 hours and