1. The sodium effluxes between the blood and the medium have been studied in Blennius pholis in 10, 40 and 100% s.w. and in transfer from 40 to 100% s.w. under equilibrium conditions with the isotope 24Na. In addition the sodium influxes have been studied in 40 and 100 % s.w. in a similar manner.

2. The total flux in 100% s.w. has been found to be 100 mM. Na/l. blood/hr., and in 40 and 10% s.w. it has been found to be 20 mM. Na/l. blood/hr.

3. The results are interpreted as showing that the presence or absence of exchange diffusion does not alter the estimation of the active sodium efflux across the gills in 100% s.w.

4. The efflux experiments showed that the animal had a rapid adaptive ability to osmo-regulate upon transference from 40 to 100% s.w.

5. The electric potential differences between the blood and external medium have been measured in animals adapted to 10, 40, 100 and 150% s.w. and the blood serum concentrations of sodium and chloride ions have been measured in these media.

6. The mean potential differences ± standard deviation in 10, 40, 100 and 150% s.w. have been found to be -3±4.5 mV., +23±4 mV., +23 ±3 mV. and +29.5±5 mV. respectively (external medium taken as reference).

7. The results are interpreted as showing that there is active outward excretion of chloride ions in animals adapted to 150, 100 and 40% s.w. and active inward absorption of chloride ions in 10% s.w. and of sodium ions in animals adapted to 40 and 10% s.w.

8. The drinking rate in 100% s.w. has been calculated to be 27 mM. Na/kg. fish /hr. or 6% body weight/hr.

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