Sodium uptake has been shown to follow saturation kinetics in many freshwater species of animals, and in the presence of hydrogen ions uptake appears to show competitive inhibition. These characteristics are compatible with the hypothesis that uptake occurs via a carrier-mediated exchange of sodium ion for hydrogen ion. However, recently it has been shown that in frog skin, sodium uptake is driven by an electrogenic pump not directly linked to sodium, and evidence is accumulating that a similar pump may occur in other freshwater animals. A mathematical model is developed that shows that a proton pump would also produce saturation kinetics and mimic carrier-mediated competitive inhibition. It would also account for the linkage between sodium influx and efflux observed in several species.