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 [3_H]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 smolt characteristics were generally similar in the Saimaa salmon and in the anadromous stocks of the Neva and Teno salmon. It is suggested that the strong smoltification process of the landlocked Saimaa salmon is somehow necessary for migration of the young salmon from river to lake and therefore has been retained in evolution.