1. The effluxes of 22Na, 36Cl and tritiated water from the euryhaline teleost, Xiphister atropurpureus were determined in 100 % sea water (480 mM-Na/kg.) and 10 % sea water.

2. Only the flux of sodium in 100% sea water contained an exchange diffusion component. The relative impermeability of Xiphister is shown by its low fluxes of sodium, chloride and water in both salinities.

3. In 10 % sea water the permeability to sodium declined while the permeability to chloride and water remained at sea water levels.

4. In both salinities diffusion and active transport are the major components of the sodium and chloride balance of Xiphister.

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