1. New experimental techniques are described for the investigation of water and electrolyte fluxes in the eel by studying the internal medium, the urine and the external medium. An experimental tank made up of two compartments isolates the water containing the head from the water containing the trunk and tail of the animal. The two water circuits are separated by remote control. Measurement can thus be made without handling the eel previously adapted to experimental conditions.

2. The freshwater eel shows low branchial exchanges and low chloride urinary losses. A positive correlation between urinary excretion of water and sodium has been shown.

3. The silver eel's skin is impermeable to water and chlorides.

4. The eel reacts to FW-SW transfer by immediately drinking water. The drinking reflex is therefore not triggered by dehydration due to the osmotic gradient.

5. During SW adaptation the eel presents a transitory hyperactivity phase of the branchial outflux corresponding to plasma hypermineralization.

6. The eel which has been adapted to sea water for 3 weeks shows the lowest chloride exchanges ever recorded among marine teleosts.

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