Some factors regulating water intake by the eel, Anguilla japonica.

1. Internal as well as external factors affecting water ingestion in the eel were analysed using oesophagus-cannulated eels. 2. Acute withdrawal of the blood induced an immediate drinking response in the freshwater eel, whereas infusion of a large amount of hypertonic saline interrupted the copious drinking observed in the seawater eel. 3. The freshwater eel responded to slow infusion of hypertonic NaCl solution by constant drinking. 4. Inhibition of drinking was observed in the seawater eel by distension of the stomach or intestine with isotonic mannitol solution. 5. The freshwater eel started drinking immediately after transfer to sea water, and stopped drinking immediately after return to fresh water. 6. Application of various salt solutions revealed that chloride ions are responsible for the induction of drinking in sea water. 7. Stimulation of drinking by chloride ions and inhibition by fresh water may be an anticipatory drinking behaviour, which facilitates adaptation of eels to both sea water and fresh water.