In order to investigate the role of nitric oxide (NO) in hypoxic tissue damage in newborns, we studied the effects of systemic administration of an inhibitor of NO synthase, N-G-nitro-L-arginine (L-NNA), and the precursor for the synthesis of NO, L-arginine (L-ARG), on the biochemical and histological changes in brain, heart, lung, liver, kidney, intestine, and skeletal muscle tissues. Four groups of 1-day-old Wistar rat pups were used: control, hypoxic, L-ARG, and L-NNA groups. L-ARG 100 mg/kg or L-NNA 2 mg/kg was administered as a bolus intraperitoneally 1.5 h before hypoxia. Hypoxia increased lipid peroxidation in all tissues except muscle; this increase was prevented by L-NNA and L-ARG in brain, heart, lung, kidney, and liver tissues. L-NNA in intestine and L-ARG in muscle tissue increased lipid peroxidation. The tissue-associated myeloperoxidase activity was decreased in the liver by L-NNA and L-ARG. Histopathological changes in intestines were villous epithelial separation and hyperemia in hypoxic and L-NNA groups which were not observed in control and L-ARG groups. In lungs, pulmonary hemorrhage was observed only in the hypoxic group. These data suggest that NO acts both as a destructive and a protective agent in the pathogenesis of hypoxia-reoxygenation injuries. Copyright (C) 2000 S. Karger AG. Basel.