We investigated the role of complement activation on the resolution of acute ischemic renal failure in the rat. Acute renal failure was induced by clamping of the renal arteries of Sprague-Dawley rats for 45 minutes (Day 0). On subsequent days, groups of rats with acute renal failure were exposed to daily zymosan infusion (an activator of the complement system), or to blood incubated with cuprophane (CUP) or polyacrylonitrile (PAN) dialysis membranes. We serially measured the change in BUN daily, glomerular filtration rate and 24-hour proteinuria on Day 3 and Day 5 following ischemia. On Day 6, the animals were sacrificed and their kidneys examined histologically. Zymosan and cuprophane exposed rats had a significant delay in the recovery of renal failure, reduced glomerular filtration rate, and histologically had more neutrophil infiltration than control or PAN exposed animals. To investigate the potential pathophysiology of these observations, we assessed the response of zymosan-exposed rats to infusion of deferoxamine (DFO), a potent inhibitor of hydroxyl radical formation (OH.). Infusion of DFO prior to zymosan significantly improved recovery of renal function. We also measured urinary thromboxane B2 levels in these groups of rats. While the groups of rats exposed to zymosan had the highest levels of thromboxane B2, these levels were not different between the groups exposed to zymosan alone, or to zymosan and DFO. These observations suggest a role for hydroxyl radicals in the prolongation of renal failure in this model. Taken together, these findings may have implications for the dialytic intervention in patients with acute renal failure.