BACKGROUND: Omega-3 fatty acids (omega-3 FA) have been demonstrated to have anti-inflammatory properties, postulated to occur through several principal mechanisms, including (1) displacement of arachidonic acid from the cellular membrane; (2) shifting of prostaglandin E(2) and leukotriene B(4) production; and (3) molecular level alterations including decreased activation of nuclear factor kappa B and activator protein-1. An additional regulator that is likely associated is the production of nitric oxide (NO) by nitric oxide synthetase. NO is a short-lived free radical involved in many biological functions. However, excessive NO production can lead to complications, suggesting that decreased NO production is a potential target for some inflammatory diseases. We hypothesized that pretreating with an omega-3 FA lipid emulsion would decrease the production of NO in macrophages and that this effect would occur through alterations in inducible nitric oxide synthetase (iNOS). MATERIALS AND METHODS: Greiss reagent was used to assess NO production in RAW 264.7 macrophages following omega-3 or omega-6 FA treatment alone or in combination with lipopolysaccharide (LPS) stimulation for 12 h/24 h. iNOS levels were determined by Western blot. Tumor necrosis factor-alpha levels were determined by enzyme-linked immunosorbent assay. RESULTS: Following LPS-stimulation, omega-3 FA pretreatment at 12 and 24 h produced significantly less NO (P < 0.05) compared to omega-6 FA or media-only conditions. omega-3 FA pretreatment at 12 and 24 h also had less iNOS protein expression compared to omega-6 FA or media-only conditions. Tumor necrosis factor-alpha production was significantly decreased with omega-3 FA treatment compared to omega-6 FA treatment (P < 0.05) after 24 h LPS stimulation. CONCLUSION: These experiments demonstrate that, in addition to other anti-inflammatory effects, omega-3 FA lipid emulsions also significantly lower NO production in LPS-stimulated macrophages through altered iNOS protein expression.