Electron Paramagnetic Resonance (EPR) oximetry was used to measure tissue oxygen tension (pO2-partial pressure of oxygen) simultaneously in the kidney cortex and outer medulla in vivo in mice. pO2 in the cortex region was higher compared to that in the outer medulla. An intravenous injection of endotoxin resulted in a sharp drop in pO2 in the cortex and an increase in the medulla region, resulting in a transient period of equal pO2 in both regions. In control kidneys, functional Magnetic Resonance (MR) images showed the cortex region to have high signal intensity (T2*-weighted images), indicating that this region was well supplied with oxygenated hemoglobin, whereas the outer medulla showed low signal intensity. After administration of endotoxin, we observed an immediate increase in signal intensity in the outer medulla region, reflecting an increased level of oxygenated blood in this region. Pretreatment of mice with NG-monomethyl-L-arginine prevented both the changes in tissue pO2 and distribution of oxygenated hemoglobin, suggesting that localized production of nitric oxide has a critical role to play in renal medullary hemodynamics. In combining in vivo EPR with MR images of kidneys, we demonstrate the usefulness of these techniques for monitoring renal pO2 and changes in the distribution of oxygen.