1. The rate of water exchange in the salmon egg was determined by means of the electromagnetic diver-balance.
2. Salmon eggs treated with alcohol to remove the plasma membrane were used to determine the diffusion coefficient, D, for water in the cytoplasm, and untreated eggs to determine the permeability coefficient, E, for water through the plasma membrane.
3. It was found that D is of the same order as that observed in other eggs, whereas the value of E is somewhat lower, but still within the same order of magnitude.
4. The results do not confirm the classical notion that the salmon egg is impermeable to water, but they show that the exchange is extremely slow, the half time being about 24 hr. In order to explain this result the contribution of egg size and environmental temperature must be taken into account.
5. It is concluded that the actually observed rate of swelling corresponds to that anticipated from the recorded permeability coefficient, and that there is therefore no reason to presume that any kind of osmoregulation takes place.