1. The mechanisms of sodium balance in Gammarus duebeni and G. pulex, adapted to various external concentrations, were compared.
2. G. duebeni could be adapted to live in 1 mm/l. NaCl solution and, in some cases, to concentrations down to 0.2 mM/l. G. pulex could survive in concentrations as low as 0.06 mM/l.
3. The sodium loss rate in G. duebeni adapted to 2% sea water was much higher than in G. pulex but was reduced to about the same level when the animals were adapted to low external concentrations.
4. In both species there was a non-linear relationship between sodium influx and the external sodium concentration. In G. duebeni the uptake mechanism was saturated at an external concentration of about 10 mM/l., whereas in G. pulex saturation was reached at a much lower concentration. The maximum rate of uptake was greater in G. duebeni than in G. pulex.
5. In both species adaptation to low concentrations involved a small increase in the sodium influx and a reduction in the loss rate.
6. The most important factor in the superiority of G. pulex over G. duebeni in surviving at low external concentrations is the high affinity for sodium displayed by the uptake mechanism in G. pulex.