Aim To investigate the effect of high density lipoprotein (HDL)on the production of nitric oxide (NO)inhibited by oxidized low density lipoprotein (ox-LDL)in vascular smooth muscle cells.Methods The production of NO in cultured human vascular smooth muscle cells was analyzed by measur-ing nitrite in media with GRIESS reagent.The im-munochemical staining was used to observe the level of inducible nitric oxide synthase(iNOS).The content of mRNA for iNOS was determined by reverse tran-scription-polymerase chain reaction(RT-PCR).Results The ox-LDL could decrease the content of iNOS and level of mRNA for iNOS in vascular smooth muscle cells induced by lipopolysaccharide and inter-feron-γ,leading to inhibition of NO production.However,the addition of HDL before incubation with LDL could increase the production of NO and level of iNOS.0. 25g/L HDL could significantly increase the release of NO,and at concentration of 1.5g/L the production of NO was 4.5 limes higher than the con-trol.The effect of HDL was concentration dependent.Conclusions HDL could inhibit the activity of ox-LDL on production of NO in vascular smooth muscle cells.