1. Sodium influx and net sodium uptake have been measured for the crayfish, Astacus pallipes, in solutions of a variety of sodium salts.
2. The nature of the anion has no effect on the sodium influx.
3. Sodium influx is related to the external sodium concentration in the same way whether the anion is chloride or sulphate.
4. The net sodium uptake rate from sodium sulphate is 65-100% of that from sodium chloride solutions.
5. In sodium sulphate solutions the sulphate ion does not penetrate.
6. There is no increase in ammonia excretion during net sodium uptake from sodium sulphate solutions.
7. An analysis of the external solution by conductivity and ion exchange showed that during sodium uptake sodium ions are not exchanged for metallic cations and must be exchanged either for ammonium or hydrogen ions without increasing their normal rate of excretion.
8. Sodium uptake may sometimes exceed ammonia production so that hydrogen-ion exchange may be involved in these cases.
9. The possibility is discussed that sodium ions exchange for either ammonium of hydrogen ions depending on the metabolic activity of the animal.