1. Intake of food, water and electrolyte by ducks maintained on fresh water and on hypertonic saline were measured over periods up to several months.

2. Transfer to saline approximately equivalent to 60% sea water was followed during the first 24 hr. by a sharp rise and fall in the plasma concentrations of sodium and chloride, which thereafter remained similar to the concentrations found in the freshwater-maintained birds.

3. Transfer to saline equivalent to 100% sea water resulted in a rise in these concentrations during the first 10 hr., which continued for a period up to 14 days, after which the birds either died or became unhealthy.

4. Upon transfer to saline drinking water (284 mM/l. Na+, 6.0 mM/l. K+) there was a gradual loss of body weight accompanied by a reduction in the food and water intake. Body weights tended to become stable after about 3 weeks, but some individuals continued to lose weight while others regained what they had lost.

5. When the concentration of sodium chloride in the drinking water exceeded 143 mM/l. the amount of sodium chloride ingested remained constant. Thus there was progressive decline in the volume of water drunk as the concentration increased. It would appear therefore that the saline-adapted duck possessed some mechanism whereby the daily intake of sodium chloride was regulated.

6. The cloacal output from saline-adapted ducks over a 24 hr. period showed that only 10% of the ingested sodium was excreted via this pathway as compared with over 70% of the ingested potassium. Most of the sodium appeared to be excreted via the nasal glands.

7. The possible interactions between the renal and extra-renal excretory pathways in the maintenance of homeostasis during adaptation to diets including hypertonic saline or seawater are discussed.

This research was supported by grants to W.N.H. from the National Science Foundation (grant no. GB 3896) and the Committee on Research, University of California.