Methods: The clinical trial will include 324 patients with GFR higher or equal to 30 ml/min who will receive intravenous contrast (Iohexol: 300 mg of l/ml; approximate volume: 120 ml at 2-5ml/second) in the following 12-24 hours. Patients with inclusion criteria, and having previously signed a consent form, will be randomised into 3 groups (intravenous hydration, oral and no hydration). Intravenous hydration will be carried out with bicarbonate 1/6M one hour before the test (3ml/Kg/h) and oral hydration with Casen solution in will be carried out 4 hours before (75 ml/10 kg) the intravenous contrast administration. In relation to the hydrated, it should be pointed out that, the same procedure would have been carried out without the study. The follow-up of renal function will be carried out after measuring creatinine and cystatin C in blood and GFR pre- and 24 hour after the test. It will also determine neutrophil gelatinase-associated lipocalin (N-GAL), IL-8 and F2-Isoprostanes levels in urine and superoxide dismutase activity in erythrocytes, pre- and in different post- test times.