The effect of N-LDL, AC-LDL and OX-LDL on the production of nitric oxide (NO) and expression of nitric oxide synthase (NOS) in mouse peritoneal macrophages stimulated by LPS was studied by measuring nitrite in media and mRNA for NOS in macrophages. The results showed that OX-LDL could inhibit NO production in macrophages stimulated by LPS, but N-LDL and AC-LDL could not. The inhibitory effect of OX-LDL was enhanced with the increase of the degree of oxidation of LDL and was time and concentration dependent. Results obtained from slot hybridization showed that OX-LDL could decrease the content of mRNA for NOS in macrophages induced by LPS, which implies that inhibition of NO production of OX-LDL was taken place at the level of transcription.