Aim To explore the change of nitric oxide in human umbilical vein endothelial cells induced by different dose of formaldehyde and the mechanisms of the nitric oxide change.Methods Human umbilical vein endothelial cells(hUVECs-12) were cultured in dulbecco's minimum essential medium(DMEM) containing 10% fetal bovine serum.At 80% confluence,cells were washed with D-Hank's fluid and cultured in DMEM containing 1% fetal bovine serum for 24 h.Cells were incubated in the presence or absence(control) of formaldehyde.In order to test the dose-effect of formaldehyde-induced endothelial cells change of nitric oxide,the cells were treated with formaldehyde at the dose of 0.0 μmol/L and 5,10,20,40,80 μmol/L for 24 h respectively.To explore the injuring mechanism of formaldehyde on endothelial cells,hydrogen peroxide(100 μmol/L),a potent free radical,was selected as a positive control;Vitamin C(100 mg/L),an anti-oxidant medicine,was selected as a protective group.After 24 h,the transudation content of lactate dehydregenase was observed by colorimetry reaction,the level of malondialdehyde in supernatant-conditioned media of cells was measured by thiobituric acid reaction(TBA) method,the level of nitric oxide in the culture medium was observed to evaluate the influence feature of formaldehyde by nitrate reductase method,the nitric oxide synthase activity was measured by biochemical method,the protein and mRNA level of inducible nitric oxide synthase and endothelial nitric oxide synthas were examined by immunohistochemistry and RT-PCR approach respectively to elucidate the injuring mechanism.Results Formaldehyde conspicuously inhibited the activity of Endothelial nitric oxide synthas,expression of endothelial nitric oxide synthas protein and endothelial nitric oxide synthas mRNA.Formaldehyde induced the activity of inducible nitric oxide synthase,expression of inducible nitric oxide synthase protein and inducible nitric oxide synthase mRNA,and increased the production of nitric oxide,malondialdehyde and the transudation content of lactate dehydrogenase on Human umbilical vein endothelial cells in dose-dependent manner;Vitamin C inhibited the activity of inducible nitric oxide synthase,expression of inducible nitric oxide synthase protein and inducible nitric oxide synthase mRNA,and increased the activity of endothelial nitric oxide synthas.Conclusion Formaldehyde can conspicuously cause change of nitric oxide on human umbilical vein endothelial cells when the concentration is between 5 and 80 μmol/L.This change is related to reducing the activity and expression of endothelial nitric oxide synthas,and increasing the activity and expression of inducible nitric oxide synthase.