N-acetylcysteine has been reported to lower serum creatinine in normal individuals. The mechanism of this effect is unknown but possible stimulation of tubular secretion of creatinine has been hypothesized based upon a lack of effect on Cystatin C levels. If this effect also occurs in subjects with chronic kidney disease, interpretation of clinical trials using N-acetylcysteine for prophylaxis of acute kidney injury would be confounded. To answer whether such confounding occurs, 50 patients with stable chronic kidney disease (Stage 3-5) will be given 4 doses of N-acetylcysteine, 1200 mg each, with measurement of serum creatinine and Cystatin C before, 4 hours and 48 hours after the last dose. No other interventions, including changes in medications, will be permitted during the study.