1. Resealed ghosts were prepared containing much potassium, very little sodium, and adenosine triphosphate (ATP), adenosine diphosphate (ADP) and (32)P-labelled orthophosphate (P(i)) at concentrations such that the ratio [ATP]/([ADP].[P(i)]) was low. Iodoacetate, iodoacetamide or both were also present. The ghosts were incubated in high-sodium, potassium-free media with and without ouabain, or in high-potassium media, and the incorporation of (32)P into ATP and ADP in 15 min was measured.2. There was some incorporation of (32)P into the nucleotides whatever the medium, possibly because of the residual activity of glycolytic enzymes, but in every experiment there was extra incorporation when the ghosts were in a high-sodium, potassium-free medium. This extra incorporation was largely abolished by ouabain (5 experiments) and partly abolished by oligomycin (1 experiment).3. It seems that if conditions are such that the over-all reaction associated with transport ATPase activity leads to an increase in free energy, the transport system will run backwards at a measurable rate and ATP will be synthesized at the expense of energy derived from ionic concentration gradients.4. The nature of the transport system is discussed in the light of the findings of this paper and the four preceding papers.