EPR spectroscopic techniques have been developed for the measurement of oxygen and nitric oxide in vivo. Specifically, the methods for in vivo measurement of these molecules has been applied to the study of septic shock, utilising an experimental murine model developed in our laboratory. Oxygen was measured as pO2 by the particlulate probes Gloxy and LiPc, which were surgically implanted at specific sites in tissues, and the soluble probe Trityl, which was administered intravenously. Nitric oxide was measured as the NO-Fe-(DETC)2 complex after administration of Fe2+ and DETC. LPS was seen to significantly decrease liver oxygen measured across the lobule and at the sinusoids by the Gloxy probe; there was a corresponding increase in nitric oxide both in the liver and systemically. The nitric oxide most likely originated from increased iNOS enzyme in the liver as demonstrated by Western blotting and the localisation of nitric oxide to the liver was confirmed with EPR imaging. LPS also caused a decrease in cerebral blood and tissue oxygenation, the rate of which was found to be dependent on the blood oxygenation. The development and applications of these in vivo EPR techniques for biomedical research and diagnostics is discussed.