1. The sodium balance mechanism of Gammarus lacustris in fresh water is virtually identical with that found in G. pulex.

2. The sodium transporting system at the body surface has a very high affinity for sodium ions. The system is half-saturated at an external concentration of about 0.14 mM/l. and fully saturated at about 1 mM/l. sodium.

3. The lowest external concentration at which sodium balance was maintained was 0.06 mM/l.

4. Both the total sodium loss rate and the sodium influx rate remained approximately constant in animals acclimatized to the range of external concentrations from 2 to 0.3 mM/l. NaCl. At lower concentrations the loss rate was reduced and the influx increased by a factor of about 1.5.

5. Changes in the sodium influx and loss rates are very closely linked together, and it is shown how these changes are related to the external sodium concentration.

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