Patients developing kidney failure after open heart surgery experience an abrupt decrease in blood flow to the kidney. The investigators hypothesize that administration of fenoldopam mesylate (a drug that increases blood flow to the kidney) to patients early in the course of their disease could reduce progression to dialysis-dependent acute renal failure. The investigators also hypothesize that restoring blood flow could induce additional injury to the kidney through the release of reactive oxygen species. Therefore, patients in this protocol will be randomized to receive a fenoldopam or the anti-oxidant MESNA. The investigators hypothesize that combination treatment with Fenoldopam and MESNA will decrease the incidence of death or dialysis at 21 days in patients with early post-operative acute renal failure.