Respiration-driven Na+ transport from Escherichia coli cells and right-side-out membrane vesicles is strictly dependent on K+. Cells from an E. colic mutant deficient in three major K+ transport systems were incapable of accumulating K+ or expelling Na+ unless valinomycin was added. Membrane vesicles from an E. coli mutant from which the genes encoding the two known electrogenic Na+/nH+ antiporters nhaA and nhaB were deleted transported Na+ as well as did vesicles from wild-type cells. Quantitative analysis of Delta psi and Delta pH showed a high driving force for electrogenic Na+/nH+ antiport whether K+ was present or not, although Na+ transport occurred only in its presence. These results suggest that an Na+/nH+ antiporter is not responsible for the Na+ transport. Respiration-driven efflux of Na+ from vesicles was found to be accompanied by primary uphill efflux of K+. Also, no respiration-dependent efflux of K+ was observed in the absence of Na+. Such coupling between Na+ and K+ fluxes may be explained by the operation of an Na+, K+/H+ antiporter previously described in E. coli membrane vesicles (Verkhovskay, M.L., Verkhovsky, M.I. and Wikström, M. (1995) FEBS Lett. 363, 46-48). Active Na+ transport is abolished when delta mu H+ is eliminated by a protonophore, but at low concentrations the protonophore actually accelerated Na+ transport. Such an effect may be expected if the Na+, K+/H+ antiporter normally operates in tight conjunction with respiratory chain complexes, thus exhibiting some phenomenological properties of a primary redox-linked sodium pump.