1. Analyses have been made of the blood and urine of the euryhaline sturgeon Acipenser medirostris and the rates of turnover of sodium and water in both sea water and fresh water have been measured.

2. The blood concentration is rather lower in fresh water than in sea water and the concentration of magnesium ions declines markedly.

3. The rate of turnover of sodium ions is high in sea water and similar to that of marine teleosts. The rate of turnover of sodium is much lower in fresh water but adaptation to fresh water is slow and the animals are more permeable to sodium than are teleosts.

4. The rate of turnover of tritiated water is more rapid in fresh water than in sea water but in each medium it is similar to that of teleosts of a similar size.

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