Changes in renal blood flow after acetylcysteine and/or contrast administration will be assessed by measuring plasma PAH clearance. Endothelin−1 (ET−1) is a potent endogenous vasoconstrictor and increased urinary ET−1 has been associated with the development of RCIN. Changes in renal ET−1 production will be measured following acetylcysteine and/or contrast by measuring ET−1 concentrations in plasma and urine and calculating its fractional excretion. Renal tubular function will be assessed by measuring the kidney's fractional excretion of sodium and potassium. Reductions in fractional excretion of these ions will supply information on renal perfusion and tubular function, and have been noted previously in RCIN. We will also measure urine levels of proteins released from damaged tubular cells.