In the acute phase of ischemic renal failure, the severe depression of the glomenrlar jiltration rate (GFR) is due to obstruction of the tubules by cells and cell debris rejectedfrom the proximal tubules, a blockade which can be prevented at least partly, by treatment with osmotic diuretics. The isosthenuria, the second typical sign in ischemic acute renal failure, probably derives from the medullary ischemia that results from an intracapillary trapping of red cells. This, in turn, is suggested to be caused by oxygen-derived free radicals, which via increasing the capillary macromolecular permeability result in a massive extravasation of plasma and hence in hemoconcen-tration. As expectedfrom this hypothesis, scavengers may ameliorate both the trapping and the consequent medullary ischemia. Unfortunately, however, a therapy using both osmotic diuretics and scavengers fails to improve the long-term outcome. Hemodilution would seem more promising, since it will both prevent the medullary ischemia seen in the acute phase and substantially improve the long-term outcome. At a hematocrit of 0.30, rat kidneys exposed to 45-min ischemia will show a GFR I month after the insult of more than 50% of the normal GFR as against 15% in untreated animals.