Microglial cells/brain macrophages from neonatal rats were examined for their capacity to generate nitrite, a product of the NO pathway. Upon incubation with bacterial lipopolysaccharide (LPS) or rat interferon-gamma (IFN-gamma), cells from the microglia-enriched fraction released measurable amounts of nitrite into the supernatant within 24-48 hr. The production of nitrite was dependent on the cell number and the dose of IFN-gamma and LPS. It could be inhibited by NG-monomethylarginine. We conclude that activated microglial cells can secrete nitrite. Stimulation of the NO pathway in microglial cells may be relevant to the pathogenesis of inflammatory and autoimmune demyelinating diseases of the brain.