1. The effect of external potassium ions on the extrusion of sodium ions by the seawater-acclimated fat sleeper, Dormitator maculatus, was investigated.

2. Removal of external potassium ions reduced the efflux of sodium from the fish by 22% while addition of 10-4 M ouabain reduced the efflux of sodium ions by 14%.

3. Addition of potassium ions to distilled-water baths into which fish were rapidly transferred stimulated sodium extrusion in a manner which could be described by the Michaelis-Menten equation. The Km of this potassium-stimulated sodium efflux was approximately 2 mM-K/1.

4. The calculated rate of sodium extrusion was 10 times the oral ingestion of sodium ions.

5. Biochemical assays of the levels of the enzyme Na-K-activated ATPase extracted from gill tissue determined that seawater-acclimated fish had 3 times the enzymic activity that fish acclimated to freshwater had.

6. In vitro potassium stimulation of the extracted Na-K-activated ATPase showed Michaelis-Menten kinetics with a Km of approximately 2 mM-K/l.

7. It is concluded that the extrusion of sodium ions by Dormitator maculatus acclimated to sea water is coupled with potassium uptake and is mediated by the enzyme Na-K-activated ATPase.

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