This experiment, using cultured bovine pulmonary artery endothelial cells (BPAEC), was undertaken to investigate roles of endogenous ONOO- in lipopolysaccharide (LPS)-caused injury to endothelial cells. The fluorescent intensity of nitrotyrosine (NT), a marker of ONOO- generation, in BPAEC represented content of endogenous ONOO- generation. The fluorescent intensity of NT and number of NT positive cells were detected with flow cytometry, and the percentage of NT positive cells was calculated. Results were as follows. (1) LPS (1 mg/L, 5 mg/L and 10 mg/L) caused marked increase in fluorescent intensity of NT in a dose dependent manner. The number and percentage of NT positive cells were markedly increased (P＜0.05). Aminoguanidine (AG), a selective inhibitor of inducible nitric oxide synthase, inhibited the increase in fluorescent intensity of NT in BPAEC induced by LPS. However, the number and percentage of NT positive cells had tendency to reduce. (2) LPS caused the enhancement of MDA content and activity of LDH in cultured supernatant (P＜0.01). AG reversed the enhancement of MDA content induced by LPS (P＜0.01). In contrast, AG had marginal effect on activity of LDH. (3) LPS induced the increase in apoptotic rate in BPAEC in a dose dependent manner. Some BPAEC stained with fluorescent probe ethidium bromide showed morphological features of apoptosis with chromatin condensation and nuclear fragmentation. AG reduced the apoptotic rate and number of apoptotic cells, both of which were still higher than those of vehicle group (P＜0.05). (4) LPS inhibited mitochondrial respiration. Effect of LPS on mitochondrial membrane potential (ΔΨ) depended on the doses of LPS. 1 mg/L LPS led to a little increase in ΔΨ, while 5 mg/L and 10 mg/L LPS significantly reduced ΔΨ. In conclusion, LPS caused injury to cultured BPAEC and increased production of ONOO-. Cytotoxicity of LPS may be mediated by endogenous ONOO-.