Gill (Na+ -K+)-ATPase activity of juvenile salmon, Salmo salar L., increased “classically” from February to the end of April in experiments in two different years, and its level before transfer to seawater was correlated directly with seawater performance (survival, plsma osmotic pressure and plasma chloride levels). A thyroxine (T4) surge occurred 2–3 weeks before the peak of gill enzyme levels. After the T4 peak the fish were clearly euryhaline, but T4 levels were not correlated with seawater adaptability. They remained able to adapt rapidly to seawater for at least 1 month after the surge. Two significant increases of tri-iodothyronine (T3) occurred while the fish were in freshwater: transfer to seawater either decreased T3 and T4 or had no effect. In freshwater, plasma growth hormone (GH) levels rose sharply concomitant with the T3 peak, 2 weeks before peak gill ATPase activity. After transfer to seawater GH increased significantly, remaining high for 7–10 days, and returning to base levels after 14 days. GH appeared to play a major role in smolting and seawater adaptation.