The brain of both sexes is a major target of estradiol and a site of estrogen synthesis during development and in the adult. In addition to the classical intranuclear estrogen receptors (ERs) ER-alpha and ER-beta, we have recently identified a novel, plasma membrane-associated ER that is neither ER-alpha nor ER-beta in the brain and uterus, which we have designated "ER-X". ER-X is a developmentally regulated estrogen-binding protein that is present in wild-type, ER-alpha gene-disrupted (alphaERKO) and ER-alpha-null mice. ER-X is re-expressed after ischemic brain injury and in adult transgenic mice with Alzheimer's disease. Although ER-X shares some homology with the C-terminal region of ER-alpha, it is not an alternative splicing variant of ER-alpha and may be a new gene. ER-X mediates 17alpha-estradiol and 17beta-estradiol activation of MAPK/ERK. In contrast, ER-alpha does not elicit ERK activation but, surprisingly, is inhibitory. The potential importance of 17alpha-estradiol, the preferred ligand of ER-X, for the brain is underscored by our findings by liquid chromatography/tandem mass spectrometry that the endogenous levels of 17alpha-estradiol are significantly elevated in the postnatal day-7 and adult mouse neocortex and hippocampus, as compared with 17beta-estradiol. That there is so much more 17alpha-estradiol than 17beta-estradiol in the brain suggests that this enantiomer would be readily available to the brain. In considering estrogens for postmenopausal treatment, one should consider all the ERs present in the brain, not just ER-alpha and ER-beta, but ER-X as well, and focus on ligands such as 17alpha-estradiol that may be more selective for this ER.