1. Gammarus duebeni from brackish water was acclimatized to salinities ranging from 100% sea water down to 0.25 mM/1 NaCl at 9 °C.

2. The body water content increased from 76 to 81% body wet weight. The ratio of total body sodium/chloride increased from 1.04 to 1.52. The sodium space remained constant, equivalent to about 65 % body H2O. The chloride space decreased from about 60% body H2O down to 35% body H2O.

3. Total body potassium remained almost constant and showed only a small decrease in dilute NaCl-media. Potassium balance was maintained for several days at an external potassium concentration of 0.010-0.015 mM/1.

4. The proportion of body water in the extracellular blood space was calculated from the assumption that potassium and chloride ions were distributed in a Donnan equilibrium between the blood and intracellular spaces. The blood space was slightly smaller than the chloride space.

5. The mean intracellular concentrations of sodium, potassium and chloride were calculated. Sodium fell from 120 to 75 mM/kg cell H2O, potassium fell from 125 to 75 mM/kg cell H2O and chloride fell from 55 to 12 mM/kg cell H2O. These concentrations are similar to the concentrations found in the muscles of decapods and in the tissues of other animals.

6. About 10% of the body chloride and 93-97% of the body potassium is situated in the cells. The proportion of intracellular sodium increased from 17-18% body sodium at 100% sea water to 40-50% body sodium at 0.25 mM/l NaCl.

7. G. duebeni from three freshwater populations were acclimatized to 2 % sea water, 0.5 and 0.25 mM/l NaCl. The body surface is three times more permeable to potassium than it is to sodium and chloride. Potassium balance in starved animals was achieved at 0.010-0.015 mM/l K. Fed animals had a higher body sodium and chloride content than starved animals.

8. The regulation of body water and ions in animals from the freshwater populations was essentially the same as in animals from brackish-water populations. The significance of the results is discussed in relation to the process of adaptation to fresh water.

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