Current models of the Na pump reaction cycle have ATP binding with low affinity to the K-occluded form and accelerating K-deocclusion, presumably by opening the inside gate. Implicit in this picture is that ATP binds after closing the extracellular gate, and thus predicts that ATP binding and extracellular cation binding would be mutually exclusive. We tested this hypothesis. Accordingly, we needed a cation that bound outside and not inside and we determined that tetrapropylammonium (TPA) behaves as such a cation: TPA competed with K (and not Na) for ATPase, TPA was unable to prevent EP formation even at low Na, and TPA decreased the rate of phosphoenzyme hydrolysis in a K⁺-competitive manner. Having established that TPA binding is a measurement of extracellular access, we next determined that TPA and inorganic phosphate (Pi) were not mutually exclusive inhibitors of pNPPase activity, implying that when Pi is bound, the transport site has extracellular access. Surprisingly, we found that ATP and TPA also were not mutually exclusive inhibitors of pNPPase activity, implying that when the cation transport site has extracellular access ATP can still bind. This is consistent with a model in which ATP speeds up the conformational changes that lead to intracellular or extracellular access, but that ATP binding is not, by itself, the trigger that causes opening of the cation site to the cytoplasm.