Acute kidney injury (AKI) is a serious complication after cardiovascular surgery. Pharmacologic interventions to prevent postoperative AKI have been, in general, unsuccessful. N-Acetylcystein (NAC) is a scavenger of reactive oxygen species that prevents renal dysfunction caused by radiocontrast agents but its ability to prevent AKI in cardiovascular surgery remains controversial. Lack of dose standardization and route of administration contributes to conflicting results in the literature. ln a earlier prospective, double-blind, placebo-controlled study 1 the investigators showed that high-dose NAC reduced the incidence of AKI in patients undergoing coronary artery bypass graft surgery. In this research the investigators aim to validate the results of the abovementioned work in a larger group of patients undergoing cardiac valve replacement, using the same protocol. Methods: This will be a unicentric, prospective, double-blind, placebo-controlled investigation. The Institutional Review Board has already approved the study protocol and all participants will sign a written informed consent. Adult patients of both sex scheduled to undergo elective cardiac valve replacement (aortic and/or mitral) at the Heart Institute, University of São Paulo Medical School, will be considered for inclusion. Exclusion criteria: patients younger than 18 years, dialysis, and participation in other studies, allergy to NAC, active infection, emergency cardiac surgery and malignancies. One-hundred fifty-four patients will be randomly allocated (1:1 ratio) to receive either NAC (Fluimucil, Zambon Laboratories, São Paulo, Brazil) 150 mg/kg in 500 mL 0.9% IV saline in 2 hours, started 2 hours before surgery, followed by NAC 50 mg/kg in 500 mL 0.9% IV saline over 6 hours (NAC group) or 0.9% IV saline (control group) at the same volume. This dose was selected because it is the highest dose sanctioned for clinical use 2 and because it has been shown to reduce the oxidative burst response to Cardiopulmonary Bypass (CPB). Allocation will be based on random computer generated numbers. Patients and investigators will be blinded to treatment assignment. Blood samples will be collected 24 hours before surgery and up to 1 week postoperatively. Serum neutrophil gelatinase associated lipocalin (NGAL) and thiobarbituric acid reactive substances will be determined at the same intervals.2 The primary outcome is the incidence of AKI, as defined by the KDIGO and Acute Kidney Injury Network classification 3 , stages 1, 2, or 3, in the first 72 hours after surgery. The secondary outcomes are death by any cause, cardiovascular events (myocardial infarction, stroke, heart failure, and life-threatening arrhythmia), time in the intensive care unit, time of hospitalization and need of dialysis. The investigators will determine thiobarbituric acid reactive substances to verify if changes in renal function would coincide with changes in oxidative stress and NGAL as an additional marker of renal damage. Patients will be followed up until death or hospital discharge. The renoprotective effect of NAC is attributed to its ability to attenuate the oxidative stress burst associated with cardiac surgery and Cardiopulmonary BYPASS (CPB). The present investigation pretend not only to address this problem, but also to offer evidence that NAC abolishes the increase in circulating reactive oxygen species thus giving a plausible explanation for the renoprotective effect of NAC. Given the clinical relevance of AKI associated to cardiac surgery, lack of dependable methods to prevent that complication and the reduced number of prospective, randomized studies addressing the problem, this work has potential practical relevance.