1. The rate of loss of sodium, chloride and water via the urine and the rate of intake of sodium, chloride and water by ingestion of the medium was determined for the euryhaline teleost, Xiphister atropurpureus.
2. The urinary losses of sodium and chloride were approximately 0.5 mM/kg. fish/day in both 100 % sea water (480 mM-Na/kg.) and 10% sea water. The ingestion of sodium and chloride by drinking the medium amounted to approximately 4 mM/kg. fish/day in 100% sea water and approximately 0.1 mM/kg. fish/day in 10% sea water.
3. The low rate of urine flow in 10 % sea water and the low drinking rate in 100 % sea water indicate a relative impermeability to water in both salinities.