The purpose of this study was to characterize the protective effect of NG-nitro-L-arginine methyl ester (L-NAME), an inhibitor of nitric oxide synthase, on oxidative stress-induced endothelial cell injury. Intracellular oxidative stress was induced by 1-chloro-2,4-dinitrobenzene, a glutathione (GSH) depleting agent, and the leakage of intracellular lactate dehydrogenase was measured as a marker of cell injury. Addition of 1-chloro-2,4-dinitrobenzene (100-500 microM) induced leakage of lactate dehydrogenase from endothelial cells, and the leakage of lactate dehydrogenase was strongly attenuated by L-NAME, but not by NG-methyl-L-arginine, also an inhibitor of nitric oxide synthase. However, cell injury induced by the Ca2+ ionophore ionomycin was not affected by L-NAME or NG-methyl-L-arginine. Moreover, neither L-NAME nor NG-methyl-L-arginine affected GSH depleting agent-induced or H2O2-induced cell injury in a rat foetal lung fibroblast cell line which lacks nitric oxide synthase. These results suggest that the protective effect of L-NAME is likely to be related to nitric oxide synthase, while the inhibition of nitric oxide production may not be involved in the protective effect of L-NAME, since NG-methyl-L-arginine did not affect endothelial cell injury.