An iron-mediated oxidative stress caused by an increase of the intracellular pool of low molecular weight complex of iron (LMWC) can be observed with iron overloading or ethanol metabolism. The aim of this study was to determine whether nitric oxide (NO) behaved as a pro-oxidant or an antioxidant in such an iron-mediated oxidative stress in rat hepatocytes. The cells were set up in primary cultures and incubated with lipopolysaccharide (LPS) and gamma-interferon (IFN) for 18 hours to induce NO synthase and to trigger NO production. Then 20 micromol/L iron or 50 mmol/L ethanol were added. Oxidative stress was evaluated by measuring lipoperoxidation using two markers: malondialdehyde (MDA) and conjugated dienes. Simultaneously, NO production was followed by the quantitation of nitrites in the culture medium, dinitrosyl iron complexes (DNICs) and mononitrosyl iron complexes (MNICs) in intact hepatocytes. DNIC and MNIC, evaluated by electron paramagnetic resonance (EPR), corresponded to NO bound to iron-containing molecules and to free NO, respectively. In cultures preincubated with LPS and IFN before iron or ethanol addition, a net decrease of lipid peroxidation induced by either NO, iron, or ethanol was noted. Moreover, an elevation of iron-bound NO and a decrease of free NO were observed in these cultures compared with the cultures incubated with only LPS and IFN. These data support the idea that there is a relationship between the changes of NO pool and the inhibition of oxidative stress. In addition, using N(G)-monomethyl-L-arginine (L-NMMA), a NO synthase inhibitor, NO was shown to be involved in the inhibition of oxidative stress induced by iron or ethanol. Addition of the chelator of LMWC iron, deferiprone, was followed by the inhibition of the increase of iron-bound NO and the reincrease of lipid peroxidation extent, which was as high as in cultures incubated only with LPS and IFN. Thus LMWC iron appeared to be involved also in the inhibition of oxidative stress induced by NO. All the results favor the conclusion that NO acts as an antioxidant in iron-mediated oxidative stress in rat hepatocytes. NO reacted with LMWC iron to form inactive iron complexes unable to induce oxidative stress in rat hepatocytes. Thus NO played a critical role in protecting the liver from oxidative stress.