The surgeon, after accidentally ligating a ureter, asks himself: "How long can the ureter remain obstructed before the kidney is irreversibly damaged?" This question presupposes some time limit before which release of the obstruction will allow return of renal function but after which the kidney is irreparably damaged. That this supposition is erroneous was nicely demonstrated by Widén1 in experiments on dogs in which continuity of a ligated ureter was restored after varying periods of complete obstruction. He showed that renal damage was progressively greater the longer the obstruction continued. For example, after recovery from 10 days of obstruction, renal function, as measured by reduction in renal weight, and by inulin and p-aminohippuric acid clearance, was about 30% below normal. After recovery from 20 days of obstruction, the reduction was about 50%, and from 30 days, about 70%, the least reduction being 42% below normal and the greatest