Post-hypoxic-ischemic (HI) reperfusion induces endothelium and neurons to produce excessive amounts of nitric oxide and superoxide, leading to peroxynitrite formation, release of protein-bound metal ions (i.e.iron), and cytotoxic oxidants. We produced severe HI in 18 newborn lambs and serially determined plasma prooxidants (non-protein-bound iron), lipid peroxidation (malondialdehyde), and antioxidative capacity [ratio of ascorbic acid/dehydroascorbic acid (AA/DHA), α-tocopherol, sulfhydryl groups, allantoin/uric acid ratio, and vitamin A] in blood effluent from the brain before and at 15, 60, 120, and 180 min after HI. The lambs were divided in three groups: six received a placebo (CONT), six received low dose (10 mg/kg/i.v.)Nω-nitro-L-arginine (NLA-10) to block nitric oxide production, and six received high dose NLA (40 mg/kg/iv; NLA-40), immediately after completion of HI. Non-protein-bound iron increased in all groups after HI but was significantly lower in both NLA groups at 180 min post-HI (p< 0.05), the AA/DHA ratio showed a consistent decrease in CONT (at 60 min post-HI,p< 0.05), but remained stable in NLA lambs. α-Tocopherol decreased steadily in the CONT, but not in the NLA lambs [180 post-H: 1.9 ± 0.9versus4.2 ± 0.7 μM(NLA-40),p< 0.05). Malondialdehyde was significantly higher in CONT lambs 120 min post-H compared with NLA groups [0.61 ± 017versus0.44 ± 0.05 μM (NLA-40),p< 0.05]. Vitamin A and sulfhydryl groups did not differ among groups. We conclude that post-H inhibition of nitric oxide synthesis diminishes non-protein-bound iron increment and preserves antioxidant capacity.Abbreviations: AA/DHA ratio,ascorbic acid/dehydroascorbic acid ratio;CONT,control;HI,hypoxic-ischemic;MABP,mean aortic blood pressure;NLA,Nω-nitro-L-arginine;ANOVA,analysis of variance