1. The osmotic and ionic regulatory abilities of adults of the euryhaline brown trout (Salmo trutta) have been studied in experiments roughly duplicating the stresses of migration from fresh water to the sea. Brown trout will survive indefinitely in full sea water if acclimatized to it at rates inversely related to the temperature.

2. Blood serum samples have been analysed for Na, K, Cl, total P and δ; muscle samples for Na, K, Cl and total solids. Changes in the concentrations of these constituents following transfers from fresh water to 50% and 100% sea have been studied. Transfers were made throughout the year and at temperatures of 10° C. and 20° C.

3. Following transfers to 50% sea water at 20° C. blood concentrations rose significantly above fresh-water levels, but returned very nearly to these levels after about a week. Transfer from 50% sea water to 100% sea water at 20° C. caused the same sequence of changes. Transfer to 100% sea water at 20°C. was fatal, and associated with very high serum concentrations. Many fish survived transfer to 100% sea water at 10°C., however, and showed evidence of internal concentrations returning to fresh-water levels after 10 days. The brown trout is strongly homoiosmotic on a long-term basis.

4. Both survival and regulatory ability were lower during the summer. There were also seasonal variations in the blood and muscle concentrations of fish in a given state of acclimatization.

5. Muscle concentration changes closely paralleled blood changes. Extracellular volume remained constant, so muscle concentration changes were attributable to changes in intracellular water. The muscles did not act as storage sites for sodium and potassium.

Contribution number 1989 from the Woods Hole Occenographic Institution.