The nicotinic acetylcholine receptor is a neurotransmitter-gated ion channel that mediates the rapid propagation of electrical signals at the nerve-muscle synapse. Its structure at 9 A resolution, in both the closed- and the open-channel forms, has been examined by electron microscopy of tubular crystals grown from Torpedo postsynaptic membranes. Binding of acetylcholine to open the channel causes a localized disturbance in the extracellular domain and initiates small rotations of the protein subunits, which trigger a change in configuration of alpha-helices lining the membrane-spanning pore. A method recently developed to correct in three dimensions for distortions present in the crystals, combined with improvements in the electron imaging, should eventually allow these conformational changes to be seen at atomic resolution.