The neurotransmitter acetylcholine is made in the cytosol and then transferred into synaptic vesicles, where it's concentration is more than 100 fold higher than in the cytosol. Synaptic vesicles isolated from neurons can take up additional acetylcholine if it is added to the solution in which they are suspended but only in the presence of ATP. Na+ ions are not required for acetylcholine uptake, but raising the pH of the solution in which the synaptic vesicles are suspended increases acetylcholine uptake. Furthermore, transport is inhibited consistent with all these observations. suggest a mechanism that is consistent with all these observations.