STUDIES 1 AND 2 These studies will assess the effect of acetylcysteine alone in healthy volunteers or in volunteers with stage III CKD. Volunteers will be randomised to a three−way, cross−over, double−blind comparison of matched placebo with oral and intravenous regimens of acetylcysteine. The primary outcome will be change in RBF (measured as para−aminohippuric acid [PAH] clearance). In addition, GFR (measured as inulin clearance), tubular function (fractional excretion of potassium and sodium), oxidative balance (total oxidative capacity, plasma and urinary isoprostanes), and plasma and urinary endothelin−1 will be assayed. The pharmacokinetics of acetylcysteine will be measured to compare IV and PO regimens.