The rates of uptake by Alteromonas haloplanktis of 19 metabolizable compounds and by V. fischeri of 16 of 17 metabolizable compounds were negligible in the absence of added alkali-metal cations but rapid in the presence of Na+. Only d-glucose uptake by V. fischeri occurred at a reasonable rate in the absence of alkali-metal cations, although the rate was further increased by added Na+, K+, or Li+. Quantitative requirements for Na+ for the uptake of 11 metabolites by A. haloplanktis and of 6 metabolites by V. fischeri and the characteristics of the Na+ response at constant osmotic pressure varied with each metabolite and were different from the Na+ effects on the energy sources used. Li+ stimulated transport of some metabolites in the presence of suboptimal Na+ concentrations and for a few replaced Na+ for transport but functioned less effectively. K+ had a small capacity to stimulate lysine transport. The rate of transport of most of the compounds increased to a maximum at 50 to 300 mM Na+, depending on the metabolite, and then decreased as the Na+ concentration was further increased. For a few metabolites, the rate of transport continued to increase in a biphasic manner as the Na+ concentration was increased to 500 mM. Concentrations of choline chloride equimolar to inhibitory concentrations of NaCl were either not inhibitory or appreciably less inhibitory than those of NaCl. All metabolites examined accumulated inside the cells against a gradient of unchanged metabolite in the presence of Na+, even though some were very rapidly metabolized. The transport of l-alanine, succinate, and d-galactose into A. haloplanktis and of l-alanine and succinate into V. fischeri was inhibited essentially completely by the uncoupler 3,5,3′,4′-tetrachlorosalicylanilide. Glucose uptake by V. fischeri was inhibited partially by 3,5,3′,4′-tetrachlorosalicylanilide and also by arsenate and iodoacetate.