1. Measurements were made of the regulation of the body ionic content, the fluxes of Na, Cl and water and the electrical potential across the intertidal teleost, Pholis gunnellus in 100% (410 mM-Na/1.) and 20% sea water.
2. The rates of the ion flux depended on the method of measurement. The flux of Cl is much below the flux of Na in 100% sea water. In 20% sea water the fluxes of Na and Cl are the same.
3. In 100% sea water only the Na flux has an exchange-diffusion component. There is no exchange-diffusion component of the flux of either ion in 20% sea water.
4. The electrical potential across Pholis changes from 18 mV. (inside positive) in 100% sea water to 6 mV (inside negative) in 100% sea water. Comparison with the Nernst potentials indicate that Cl is actively transported in both salinities while Na may possibly be actively transported in 100% sea water.
5. In 20% sea water the permeability to Na decreases to 51% of the permeability in 100% sea water, while the Cl permeability decreases to 82% and the water permeability remains at the sea-water level.
6. In both salinities the rate of diffusion of Na ions is greater than the rate of diffusion of Cl ions.
7. The osmotic permeability to water is approximately equal to the diffusional permeability to water in both salinities.
8. It is concluded that the passive movements of Na, Cl and water must be independent of each other and not via uncharged, water-filled pores.