The inherited retinal dystrophy observed in Royal College of Surgeons (RCS) rats is a widely used model for the study of the photoreceptor degeneration that occurs in retinitis pigmentosa and macular degeneration. The visual cell degeneration is accompanied by an abnormal accumulation of microglial cells in the retina of RCS rats presenting the dystrophy. In the present study, we show that combined stimulation of RCS dystrophic retinal Müller glial (RMG) cells with interferon-gamma (IFN-gamma) and lipopolysaccharide (LPS) induced the release in culture supernatants of significantly higher amounts of tumor necrosis factor (TNF) and nitric oxide (NO) compared to nondystrophic congenic controls. In contrast, the levels of TNF and NO found in the supernatants from microglial cells were not significantly different in both strains. Interestingly, as shown by thymidine incorporation, microglial cells from RCS dystrophic rats have a prominent capacity of proliferation in culture medium compared to microglia isolated from RCS non dystrophic controls. Incubation of RMG cells and microglia with the stereoselective inhibitor of NOS, NG-monomethyl-L-arginine (L-NMMA), inhibited nitrite release in LPS + IFN-gamma-stimulated RMG cells and microglia. The addition of TGF-beta with LPS + IFN-gamma clearly inhibited TNF release in supernatants from both dystrophic and control rat RMG cells and microglia. While TGF-beta significantly inhibited nitrite synthesis in RMG cells, the effect on nitrite synthesis by microglia was very low. The retinal dystrophy observed in RCS dystrophic rats could result from an abnormal reactivity of RMG and microglial cells to release TNF and NO in response to stimulants. The immunomodulatory cytokine TGF-beta and inhibitors of NOS could be negative regulators in the cytokine network and nitrite synthesis thus interfering with the development of photoreceptor cell death.