The effects of the antibacterial peptide PR-39 on nitric oxide (NO) and liver oxygenation (pO(2)) in a mouse model of endotoxaemia have been explored. In vivo electron paramagnetic resonance (EPR) spectroscopy was used to make direct measurements of liver NO and pO(2). Measurements of pO(2) were made at two different anatomical locations within hepatic tissue to assess effects on blood supply (hence oxygen supply) and lobule oxygenation; selectively from the liver sinusoids or an average pO(2) across the liver lobule. PR-39 induced elevated levels of liver NO at 6 h following injection of lipopolysaccharide (LPS) as a result of increased iNOS expression in liver, but had no effect on eNOS or circulatory NO metabolites. Sinusoidal oxygenation was preserved, and pO(2) across the hepatic tissue bed improved with PR-39 treatment. We propose that the beneficial effects of PR-39 on liver in this septic model were mediated by increased levels of local NO and preservation of oxygen supply to the liver sinusoids.