Technical Abstract: Walnuts are a rich source of essential fatty acids, including the polyunsaturated fatty acids alpha-linolenic acid (ALA) and linoleic acid (LA). Essential fatty acids have been shown to modulate a number of cellular processes in the brain, including the activation state of microglia. Microglial activation can result in the generation of cytotoxic intermediates and is associated with a variety of age-related and neurodegenerative conditions. In vitro, microglial activation can be induced with the bacterial cell wall component lipopolysaccharide (LPS). In the present study, we generated a methanolic extract of English walnuts (juglans regia) and examined the effects of walnut extract exposure on LPS-induced activation in BV-2 microglial cells. When cells were treated with walnut extract prior to LPS stimulation, production of nitric oxide (NO) and expression of inducible nitric oxide synthase (iNOS) were attenuated. Walnut extract also induced a decrease in tumor necrosis-alpha (TNFa) production. We further found that walnut extract induced internalization of the LPS receptor, toll-like receptor 4 (TLR4), and that the anti-inflammatory effects of walnut were dependent on functional activation of phospholipase D2 (PLD2). These studies represent the first to describe the anti-inflammatory effects of walnuts in microglia, which could lead to nutritional interventions in the prevention and treatment of neurodegeneration.