AbstractThe estrogen receptor (ER) is a member of a superfamily of ligand‐regulated transcription factors that were thought to localize primarily to the nucleus; however, a membrane‐associated ER that can initiate rapid non‐genomic cell‐signaling events has been identified recently in various cells. The presence of the ER in myelin has not been reported although the nuclear form has been detected in oligodendrocytes. We have shown that an ER with similarities to ERβ is present in isolated central nervous system (CNS) myelin, the myelin sheath in spinal cord and brain sections, and the oligodendrocyte plasma membrane using two‐dimensional (2D) PAGE, mass spectrometry, peptide mass fingerprinting, Western blotting of 1D and 2D gels, and confocal microscopy. Caveolin‐1 was also shown to be present in isolated CNS myelin and oligodendrocyte plasma membranes, where it was partially colocalized with ER. After Triton X‐100 extraction of myelin, the ER was present in an insoluble low‐density glycosphingolipid‐enriched fraction and even more in a higher density fraction also containing caveolin and cytoskeletal elements, suggesting that the membrane form of ER may be associated with caveolin or the radial component of myelin. The discovery of the ER in the oligodendrocyte plasma membrane and within the myelin sheath indicates a potential role for estrogen in myelin maintenance or functions. © 2004 Wiley‐Liss, Inc.