Bovine retinal pigmented epithelial cells (RPE cells), after activation with interferon gamma (IFN gamma) and lipopolysaccharide (LPS), express an inducible nitric oxide (NO) synthase. This activity can be inhibited by the fibroblast growth factors (FGF), FGF-1 (acidic FGF) and FGF-2 (basic FGF). We have attempted to elucidate the molecular mechanisms involved in the FGF inhibition of NO synthase activity. Analysis by immunocytochemistry and Western blot using a polyclonal antibody against the inducible NO synthase of rat liver reveals that RPE cells co-stimulated with FGF-2 and LPS/IFN gamma accumulate lower levels of NO synthase than in the absence of FGF-2. Northern blot analysis by cross-species hybridization with a mouse macrophage NO synthase cDNA probe shows that a 4.4-kb mRNA accumulates when RPE cells are activated with LPS/IFN gamma. The level of inducible NO synthase mRNA in LPS/IFN gamma-activated RPE cells is markedly reduced by FGF-1 or FGF-2 treatment. Message stability studies revealed that the presence of FGF did not accelerate mRNA degradation, implying that FGF did not act on inducible NO synthase mRNA stability, but more probably on its expression. Furthermore an effect of FGF on IFN gamma receptors was excluded, since IFN gamma binding was not altered by FGF. Since NO acts as a cytostatic compound, FGF, by preventing NO synthase expression in RPE cells, may protect the retina from endotoxin and cytokine-mediated tissue damage.