The level of NOS 11 mRNA was markedly increased during 24 h lipopolysaccharide (LPS) stimulation, but showed no further increase thereafter. On the other hand, the level of NOS 11 mRNA in J774A.1 cells transfected with an expression vector containing the rat csk cDNA (J.Csk) was significantly increased during 3 h LPS stimulation, but rather decreased thereafter. Although no significant difference was observed in the activation of NF-kappaB by LPS among parental J774A.1, J774A.1 transfected with promoterless vector (J.pBK), and J.Csk cells, activity of c-Jun N-terminal kinase (JNK) and nuclear translocation of nuclear factor activator protein-1 (AP-1) were markedly upregulated in the J.Csk cells. Then luciferase reporter vectors containing NOS 11 promoter with mutations in two AP-1-like sites (U site, -1126similar to-1120; L site, -524similar to-518) were transiently transfected in J774A.1 cells. The promoter activity following LPS stimulation for 24 h was significantly increased by mutation at the L site, but not by mutation at the U site, suggesting that NOS 11 expression is negatively regulated, at least in part, through the AP-1-like L site in response to LPS.