Abstract: The Na+-K+-ATPase enzyme activity in gill tissues, changes of plasma ions level and, survival and growth performances of rainbow trout, Oncorhynchus mykiss, were investigated after gradual and direct acclimatization to high saline water in the Aegean Sea. Five experiments were performed with rainbow trouts weighing 120, 140, 160, 200 and 225 g. In this experiment, Na+, Cl-, K+, Ca+ and Mg+2 ion levels in blood plasma, Na+-K+-ATPase enzyme activities in gill tissues of rainbow trouts weighing 160 g in both freshwater and saltwater were measured. After adaptation to seawater, fish were held in marine cages of 64 m3 volume to grow up at Island of Urla-Karantina. Rainbow trouts weighing 120, 140, 200 and 225 g were reared in these cages for 80 days. Parallel to that, fish weighing 225 g were stocked in freshwater for comparison. The best survival percentage among directly acclimatized rainbow trouts was seen in 200-225 g fish, whereas the survival percentage among gradually acclimatized trouts presented similar results in every group. It can be concluded from these studies that, rainbow trout in certain seasons (Autumn-Winter) can adapt to the Aegean Sea condition (3.6-3.7%) and show good growth performance.