DOSE JUSTIFICATION There are currently no data to guide choice of an optimum oral or IV acetylcysteine regimen. This study will assess the effects of an IV and an oral dose of acetylcysteine (IV 200mg/kg; PO 68.6 mg/kg) that are very similar to those currently used in clinical practice. The oral dose is expected to produce a plasma concentration much lower than the IV dose; however, it is believed that a first pass effect after oral administration may increase conversion in the liver of acetylcysteine into cysteine and then glutathione, increasing the efficacy of an oral dose. The first clinical trials in RCIN used an arbitrary low dose oral regimen of acetylcysteine − 600mg twice a day (BD) for 2 days, starting the day before dye administration (total dose 2.4 g, 34.3 mg/kg in a 70kg patient). Recent studies have used higher total doses of up to 1500mg BD for 2 days (total dose 6 g, 85.7 mg/kg in a 70kg patient). IV regimens have also been assessed since they have been proposed to be effective when started on the same day as the dye administration, rather than the day before. The first trial used a dose similar to that used for early treatment of paracetamol−induced hepatotoxicity - 150 mg/kg over 30 min, then 50mg/kg over 4 hrs (total dose 200 mg/kg) - and reported less nephropathy. Other studies used lower doses (eg. 500 mg over 15 mins [7.1 mg/kg in 70 kg patient] or 1000 mg twice, before and after the procedure, [28.5 mg/kg in 70kg patient]) and did not find any benefit. Overall, the choice of acetylcysteine regimen for previous studies seems to have been based on ease and prior practice in paracetamol poisoning rather than knowledge of acetylcysteine's effects on the kidney.