Nitric oxide (NO) and hydrogen peroxide (H₂O₂) show cooperativity in their cytotoxic action. The present study was performed to decipher the mechanisms underlying this phenomenon. In cultured liver endothelial cells and in cultured, glutathione-depleted hepatocytes, the combined exposure to NO (released by spermine NONOate, 1 mM) and H₂O₂ (released by glucose oxidase) induced cell injury that was far higher than the injury elicited by NO or H₂O₂ alone. In both cell types, the addition of the NO donor increased H₂O₂ steady-state levels, although with different kinetics: in hepatocytes, the increase in H₂O₂ levels was already evident at early time points while in liver endothelial cells it became evident after ≥2 h of incubation. NO exposure inhibited H₂O₂ degradation, assessed after addition of 50 μM, 200 μM, or 4 mM authentic H₂O₂, significantly in both cell types. However, again, early and delayed inhibition was observed. The late inhibition of H₂O₂ degradation in endothelial cells was paralleled by a decrease in glutathione peroxidase activity. Glutathione peroxidase inactivation was prevented by hypoxia or by ascorbate, suggesting inactivation by reactive nitrogen oxide species (NO_x). Early inhibition of H₂O₂ degradation by NO, in contrast, could be mimicked by the catalase inhibitor azide. Together, these results suggest that the cooperative effect of NO and H₂O₂ is due to inhibition of H₂O₂ degradation by NO, namely to inhibition of catalase by NO itself (predominant in hepatocytes) and/or to inhibition of glutathione peroxidase by NO_x (prevailing in endothelial cells).