1. Studies were undertaken to determine the degree of regulation of sodium, chloride and water displayed by the intertidal teleost, Xiphister atropurpureus, over a range of salinities.
2. The plasma concentrations of sodium and chloride declined by approximately 15 % in 10 % sea water (48 mM-Na/kg.) and the intracellular and extracellular spaces increased by approximately 6 % in 10% sea water.