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One-year-old Atlantic salmon improved their seawater adaptability in late spring, at a time when 2-year-old fish smoltify, though not to the same degree as the latter. One-year-old males that matured precociously were less able to adapt to seawater than immature fish. Both 1- and 2-year-old immature salmon increased their plasma levels of thyroxine when entering the smolt stage; however, both the increase and the levels were much lower in the 1-year-old fish. The seasonal patterns of plasma triiodothyronine and thyroxine were studied in immature parr and precocious males held in fresh and brackish water. The most consistent difference was that the levels of triiodothyronine and thyroxine were higher in mature males than in immature fish during winter.