Although the biological activity of estrogen is generally mediated through nuclear estrogen receptors, a large body of evidence indicates that estrogen may also affect target cells upon binding to putative membrane estrogen receptors (mER) coupled to intracellular signaling cascades; however, no agreement has been reached on the nature and precise location of the putative estrogen receptor (ER) responsible for these rapid effects. In the present report we show that the expression of ERalpha is associated with the plasma membrane fraction of rat hypothalamic tissue at embryonic day 16. Moreover, our experiments extend these results to rat hypothalamic neurons in vitro showing that ERalpha can be detected from the cell exterior as a biotinylated cell-surface protein. We have also shown that the mERalpha is under regulation of estradiol, and the ERalpha agonist, 4,4',4''-(4-propyl-[1H]-pyrazole-1,3,5-triyl)trisphenol, induced extracellular-signal-regulated kinase signaling in a dose-dependent manner and in a time-course not compatible with genomic actions, supporting the notion of a membrane-initiated phenomenon.