[en] Membrane proteins from alkalophilic Bacillus firmus RAB were extracted with octylglucoside, reconstituted into liposomes made from alkalophile lipids. The proteoliposomes were loaded with ²²Na⁺. Imposition of a valinomycin-mediated potassium diffusion potential, positive out, resulted in very rapid efflux of radioactive Na⁺ against its electrochemical gradient. That the Na⁺ efflux was mediated by the electrogenic Na⁺/H⁺ antiporter is indicated by the following characteristics that had been established for the porter in previous studies: dependence upon an electrical potential; pH sensitivity, with activity dependent upon an alkaline pH; inhibition by Li⁺; and an apparent concentration dependence upon Na⁺ that correlated well with measurements in cells and membrane vesicles