Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterized histopathologically by a loss of neurons and an accumulation of beta-amyloid plaques, neurofibrillary tangles, dystrophic neurites, and reactive glial cells. While most previous studies on the neurodegeneration of AD have focused on neuronal cells and direct beta-amyloid-mediated neurotoxicity, few have focused on the role of reactive glial cells in beta-amyloid-mediated neurotoxicity. In the present study nitric oxide release from cultured rat microglia was examined by exposing the cells to synthetic beta-amyloid peptides (beta 25-35 and beta 1-40) alone and in combination with the cytokines IFN-alpha/beta (100 U/ml), IL-1 beta (100 U/ml), TNF-alpha (100 U/ml), TNF-beta (100 U/ml), or IFN-gamma (10, 100, 500, or 1000 U/ml). Assessment of microglial release of nitric oxide was based on the colorimetric assay for nitrite in the culture medium and histochemistry for nitric oxide synthase. Of the cytokines tested, only IFN-gamma (1000 U/ml) induced nitric oxide release from microglia. beta 25-35 did not stimulate nitric oxide release by itself, but it did induce nitric oxide release when co-exposed with IFN-gamma (100, 500, and 1000 U/ml). In contrast, beta 1-40 did induce microglial release of nitric oxide by itself, and this effect was enhanced significantly by co-exposure with IFN-gamma (100 U/ml). These findings warrant a further investigation into the role of microglia in the neurodegeneration of Alzheimer's disease via nitric oxide toxicity induced by the synergistic action of beta-amyloid and a costimulatory factor.