Intact cells of the marine bacterium Alteromonas haloplanktis 214 oxidized NADH, added to the suspending medium, by a process which was stimulated by Na+ or Li' but not K+. Toluene-treated cells oxidized NADH at three times the rate of untreated cells by a mechanism activated by Na+ but not by Li' or K+. In the latter reaction, K+ spared the requirement for Na+. Intact cells of A. haloplanktis oxidized ethanol by a mechanism stimulated by either Na+ or Li'. The uptake of ot-aminoisobutyric acid by intact cells of A. haloplanktis in the presence of either NADH or ethanol as an oxidizable substrate required Na+, and neither Li' nor K+ could replace it. The results indicate that exogenous and endogenous NADH and ethanol are oxidized by A. haloplanktis by processes distinguishable from one another by their requirements for alkali metal ions and from the ion requirements for membrane transport. Intact cells of Vibrio natriegens and Photobacterium phosphoreum oxidized NADH, added externally, by an Na+-activated process, and intact cells of Vibrio fischeri oxidized NADH, added externally, by a K+-activated process. Toluene treatment caused the cells of all three organisms to oxidize NADH at much faster rates than untreated cells by mechanisms which were activated by Na+ and spared by K+.