Previous studies have shown that activation of the RON receptor tyrosine kinase inhibits inducible NO production in murine peritoneal macrophages. The purpose of this study is to determine whether inflammatory mediators such as LPS, IFN-γ, and TNF-α regulate RON expression. Western blot analysis showed that RON expression is reduced in peritoneal macrophages collected from mice injected with a low dose of LPS. The inhibition was seen as early as 8 h after LPS challenge. Experiments in vitro also demonstrated that the levels of the RON mRNA and protein are diminished in cultured peritoneal macrophages following LPS stimulation. TNF-α plus IFN-γ abrogated macrophage RON expression, although individual cytokines had no significant effect. Because LPS and TNF-a plus IFN-g induce NO production, we reasoned that NO might be involved in the RON inhibition. Two NO donors, S-nitroglutathione (GSNO) and (6)-S-nitroso-N-acetylpenicillamine (SNAP), directly inhibited macrophage RON expression when added to the cell cultures. Blocking NO production by NO inhibitors like TGF-b prevented the LPS-mediated inhibitory effect. In Raw264.7 cells transiently transfected with a report vector, GSNO or SNAP inhibited the luciferase activities driven by the RON gene promoter. Moreover, GSNO or SNAP inhibited the macrophage-stimulating proteininduced RON phosphorylation and macrophage migration. We concluded from these data that RON expression in macrophages is regulated during inflammation. LPS and TNF-α plus IFN-γ are capable of down-regulating RON expression through induction of NO production. The inhibitory effect of NO is mediated by suppression of the RON gene promoter activities.