Estrogens are well known to exert antiinflammatory effects outside the central nervous system (CNS). They have also been shown to exert neuroprotective effects in the CNS after several types of injury, including neurodegeneration. However, the molecular mechanisms by which these effects occur remain unclear. Because microglial hyperactivation and their production of neurotoxins is associated with many types of brain injury for which estrogens are beneficial, we sought to investigate the ability of estrogen to modulate microglial function. Furthermore, because little is known regarding the role of each of the two known estrogen receptors (ERs) in microglia, our studies were designed to test the hypothesis that 17beta-estradiol (E(2)) exerts antiinflammatory effects in microglia, specifically via interactions with ERbeta. We tested this hypothesis using the murine microglial cell line BV-2, which naturally expresses only ERbeta. Our results indicate that not only does E(2) decrease lipopolysaccharide (LPS)-stimulated nitric oxide (NO) production and inducible nitric oxide synthase (iNOS) expression, it also reduces the expression of cyclooxygenase-2, a target for estrogen that has not previously been reported for ERbeta. We also observed that LPS-stimulated TNFalpha mRNA was increased by estrogen. E(2) exerts these effects within 30 min compared with typical estrogen transcriptional responses. Tamoxifen and ICI 182,780 differentially blocked the inhibitory effects of E(2) on LPS-stimulated iNOS and cyclooxygenase-2. In addition, we show that E(2) alters LPS-stimulated MAPK pathway activation, supporting the idea that alterations in the MAPKs may be a potential mechanism by which ERbeta mediates decreased microglial activation.