Plasma ion values and mortality rates were compared for 450 rainbow trout (Oncorhynchus mykiss) of three sizes following direct transfer from fresh water to Black Sea water of about 18 ppt. In fish of 14.29+/-0.30 g, plasma Na+, Cl- and K+ levels significantly (p<0.05) rose above initial values five days after the transfer and peaked at 178.6+/-5.66, 153.9+/-0.14 and 1.14+/-0.04 mM/l, respectively. In 20.45+/-0.48 g fish, these values also rose significantly, reaching 172.4+/-4.24, 151.8+/-6.65 and 0.98+/-0.04 mM/l by day 5. In fish of 29.91+/-0.99 g, however, plasma Na+ and Cl- concentrations peaked 19 days after transfer, reaching only 165.5+/-6.43 and 142.9+/-8.34 mM/l, while plasma K+ reached its highest concentration of 1.02+/-0.06 mM/l on day 12, All three concentrations dropped to near initial values on day 26. In all groups, the plasma Ca2+ level rose significantly (p<0.05) above the initial value five days after transfer and then declined while the plasma P5+ concentration dropped on day 5, reaching a minimum on day 12 and recovering the initial level on day 26. The lowest mortality (8.0+/-1.89%) was recorded in the 30 g group, followed by 19.3+/-0.94% and 24.7+/-0.94% in the 20 g and 14 g groups. The failure of the smallest fish to adapt after direct transfer to sea water was likely due to excessively high plasma Na+ and Cl- concentrations and tissue dehydration, indicating that fish of 30 g best adapt to a seawater environment of 18 ppt.