In the present study the effects of 17β-estradiol on microglial activation are described. Estrogen replacement therapy has been associated with decreased severity of age-related neurodegenerative diseases such as Alzheimer’s disease, and estrogens have potent immunosuppressive properties outside of the brain. To determine the role that microglial cells might play in estrogen-mediated neuroprotection, primary rat microglia and N9 microglial cell lines were treated with increasing doses of 17β-estradiol before or during immunostimulation by lipopolysaccharide, phorbol ester, or interferon-γ. Pretreatment with 17β-estradiol, but not 17α-estradiol or progesterone, dose dependently attenuated microglial superoxide release and phagocytic activity. Additionally, 17β-estradiol attenuated increases in inducible nitric oxide synthase protein expression, but did not alter nuclear factor-κB activation. The antiinflammatory effects of 17β-estradiol were blocked by the antiestrogen ICI 182,780. Additionally, 17β-estradiol induced rapid phosphorylation of the p42/p44 mitogen-activated protein kinase (MAP kinase), and the MAP kinase inhibitor PD 98059 blocked the antiinflammatory effects of 17β-estradiol. Overall, these results suggest that estrogen receptor-dependent activation of MAP kinase is involved in estrogen-mediated antiinflammatory pathways in microglial cells. These results describe a novel mechanism by which estrogen may attenuate the progression of neurodegenerative disease and suggest new pathways for therapeutic intervention in clinical settings.