Oxidative stress during ischemia-reperfusion acute renal failure (IR-ARF) was non-invasively evaluated with in vivo electron paramagnetic resonance (EPR) imaging. Female ICR mice underwent left nephrectomy and 30 minutes ischemia-reperfusion on the right kidney. Oxidative stress was evaluated as organ reducing activity with the half-lives of the spin probe 3-carbamoyl-2,2,5,5-tetramethylpyrrolidine-1-oxyl (Carbamoyl-PROXYL) using: 1) conventional Lband EPR, which showed organ reducing activity in the whole abdominal area, and 2) EPR imaging, which showed semi-quantitative but organ-specific reducing activity. The results were compared to the reducing activity of organ homogenate and phosphatidylcholine hydroperoxide (PC-OOH) concentrations. Half-lives of Carbamoyl-PROXYL in the whole upper abdominal area, measured by L-band EPR, were prolonged on the 3rd day after ischemiareperfusion and recovered to the level of non-treated mice on the 7th day. This trend resembled closely that of serum creatinine and blood urea nitrogen concentration. The EPR imaging-measured Carbamoyl-PROXYL half-life was also prolonged on the 3rd day both in the kidney and the liver. However, in the kidney, this showed only partial recovery on the 7th day. In the liver, this convalescence was more remarkable. The ex vivo studies of organ reducing activity and PC-OOH agreed with the results from EPRI, but not with those from L-band EPR. These results indicate that renal reducing activity shows only partial recovery on the 7th day after ischemia-reperfusion, when serum creatinine and BUN have recovered. EPR imaging is an appropriate and useful method for the non-invasive evaluation of oxidative stress in the presence of renal injury.