We will take a structured 4−part approach, using randomised controlled cross−over studies to assess the effect of acetylcysteine on renal function in both normal and diseased kidneys, and then the effect of contrast on normal kidneys, with and without acetylcysteine treatment. We will also perform a parallel−group randomised controlled trial of angiography on CKD patients with and without acetylcysteine treatment. The four groups will be studied simultaneously. As per the decision of the MHRA, this study is a phase IV trial of an authorized medication rather than a phase I trial.