Estrogen treatment induces synaptic plasticity accompanied by damaged structures and aggregates of peroxidase in astrocytes in the hypothalamic arcuate nucleus of the rat. Synaptic plasticity also occurs within the arcuate nucleus after physiologic surges of estrogen. Although the function of estrogen-induced peroxidase is unclear at present, in other systems peroxidase can generate free radicals by catalyzing the oxidation of some molecules, including estrogen. Because free radicals underlie remodeling in a number of tissues, estrogen-induced free radicals could mediate synaptic remodeling within the arcuate nucleus. Although they contain estrogen-inducible peroxidase, astrocytes do not contain estrogen receptors as measured by conventional techniques, suggesting that estrogen-inducible peroxidase arises from some novel mechanism. Estrogen could induce peroxidase within receptor-deficient astrocytes by binding to receptors in neurons and stimulating the release of some factor that interacts with astrocytes. Alternatively, estrogen could act directly on astrocytes in the absence of estrogen receptors. Although astrocytes in the hypothalamus of the rat do not contain classical nuclear estrogen receptors, they do bind fluorescein-conjugated estradiol in extranuclear sites. The distribution of fluorescein-conjugated estradiol binding within the hypothalamus overlaps that of peroxidase-rich astrocytes, and double labeling reveals many cells with the stellate morphology of astrocytes, containing both peroxidase and fluorescein-conjugated estradiol binding. However, because peroxidase and fluorescein-conjugated estradiol always occupy different compartments of the cell, the fluorescein-conjugated estradiol is not binding to peroxidase.