Na+,K+-ATPase pumps three Na+ ions out of cells in exchange for two K+ taken up from the extracellular medium per ATP molecule hydrolysed, thereby establishing Na+ and K+ gradients across the membrane in all animal cells. These ion gradients are used in many fundamental processes, notably excitation of nerve cells. Here we describe 2.8 angstrom-resolution crystal structures of this ATPase from pig kidney with bound Na+, ADP and aluminium fluoride, a stable phosphate analogue, with and without oligomycin that promotes Na+ occlusion. These crystal structures represent a transition state preceding the phosphorylated intermediate (E1P) in which three Na+ ions are occluded. Details of the Na+-binding sites show how this ATPase functions as a Na+-specific pump, rejecting K+ and Ca2+, even though its affinity for Na+ is low (millimolar dissociation constant). A mechanism for sequential, cooperative Na+ binding can now be formulated in atomic detail.