Measurements were made of the flux of tritiated water across various marine, freshwater and euryhaline teleosts. The effects of temperature, body size, species differences, salinity, stress and anaesthetization were studied.

2. The Q10 of the flux of water across teleosts is approximately 1·90 and the flux is related to the 0·88 power of the body weight.

3. All of the freshwater species studied were more permeable to water than the marine species. Euryhaline teleosts appear to have about the same permeability as species to which they are most closely related.

4. While the flounder and the yellow eel are more permeable to water in fresh water than in sea water, the silver eel and the brown trout do not change their permeability and the 3-spined stickleback is less permeable to water in fresh water than in sea water.

5. While stress markedly increases the permeability to water of large brown trout, it has no effect on small brown trout and seems to decrease the water permeability of the plaice.

6. Anaesthetization has no effect on the water permeability of the goldfish but markedly increases the permeability to water of the silver eel.

7. The relationship between the flux of water and either the drinking rate in sea water or the urine flow in fresh water is discussed.

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