Classic theories on steroid hormone action have been undergoing considerable revision owing to cellular effects that occur too quickly to be mediated by gene transcription. Estrogen is no exception and plays a leading role as the story unfolds. A growing acceptance that estrogen receptors exist on the cell membrane forces an adaptation of the traditional scenario in which steroids randomly diffuse across cell membranes and bind intracellular receptors, triggering transcription. In the new version, estrogen is ferried through the bloodstream to specific addresses, where it binds cell-surface receptors and triggers signal transduction pathways more commonly associated with neurotransmitters and growth factors.