We compared smolt physiology of Atlantic salmon stocks, that migrate from freshwater stream environments to a freshwater lake (Lake Saimaa, Finland), from the Neva River (Russia) to the brackish water of the Baltic Sea, and from the Teno River (Finland/ Norway) to the full-strength seawater of the Arctic Ocean. The three fish stocks were reared under common garden conditions, and ion homeostasis, seawater tolerance, activities of gill and kidney Na,K-ATPase and [3H]ouabain binding of gill and kidney tissues were determined at different phases of smoltification. Activity of gill Na,K-ATPase and number of gill [3H]ouabain binding sites reached peak values in May and were strongly suppressed in July. Osmolality and Na+, Cl-, and Mg2+ ion concentrations of blood plasma decreased during smoltification. αla and αlb isoforms of gill Na,K-ATPase decreased and increased, respectively, during smoltification. Although small differences appeared at some points in time, the seasonal pattern and magnitude of changes in...