1. Regardless of the composition of the incubation medium, the nasal gland tissue from ducks adapted to hypertonic saline always released inorganic phosphate from the ATP in the medium at a higher rate than tissue derived from freshwater-adapted ducks.

2. Changes in the nasal gland ATP-ase activities during the period of adaptation to saline followed the same time course as the changes which occurred in the sodium-excretory capacity of the tissue.

3. Adaptation to saline resulted in a three- to fourfold increase in the weight-specific ATP-ase activity which was accompanied by a three- to fourfold increase in the weight-specific sodium-excretory capacity.

4. In the saline-adapted birds a positive correlation was found between the sodium-excretory capacity of the nasal gland tissue and the corresponding ATP-ase activity.

5. The moles of sodium excreted per mole of ATP hydrolysed was constant irrespective of whether the birds were adapted to either fresh water or salt water.

6. Concommitant with the changes in weight-specific sodium excretion and ATP-ase activity, an increase in the sodium concentration of the nasal gland fluid was observed. An approximately twofold increase in nasal gland weight also occurred during this period.

7. All changes which were observed to occur during the period of adaptation to saline were reversed when the birds were returned to fresh water.

8. The observed changes in enzyme activity and sodium excretory rates of the nasal glands are discussed in relation to possible cellular changes which might occur and their importance relative to the birds's ability to adapt to a marine environment.

This work was supported by research grants to W.N.H. from the National Science Foundation (Grant no. GB3896) and the Commmittee on Research, University of California.