The potential roles of growth hormone (GH) and insulin-like growth factor I (IGF-I) in seawater (SW) acclimation of juvenile Atlantic salmon (Salmo salar) were examined. Compared to controls, fish in 12 ppt seawater given one or three injections (2–6 days) of GH (ovine, 0.2μg · g⁻¹) or IGF-I (recombinant bovine, 0.05–0.2μg · g⁻¹) had significantly greater salinity tolerance as judged by lower plasma sodium, osmolality, and muscle moisture content following transfer to 34 ppt. Single injections of GH and IGF-I in fish in fresh water failed to improve salinity tolerance following transfer to 25 ppt SW. Treatment of fish in 12 ppt with GH or IGF-I for 2–6 days did not increase gill Na⁺, K⁺-ATPase activity, but treatment with GH prevented decreases in gill Na⁺, K⁺-ATPase activity that occurred in controls following transfer to 34 ppt seawater. Fish in fresh water administered GH by implants (5.0μg · g⁻¹) or osmotic minipumps (0.5μg · g⁻¹day⁻¹) for 7–14 days had greater gill Na⁺, K⁺-ATPase activity and salinity tolerance than controls. IGF-I administered by implants (0.5–1.0μg · g⁻¹) or osmotic minipumps (0.1μg · g⁻¹day⁻¹) for 4–14 days did not increase salinity tolerance or gill Na⁺, K⁺-ATPase activity. Cortisol implants (50μg · g⁻¹) also increased gill Na⁺, K⁺-ATPase activity and salinity tolerance after 14 days, and in combination with GH had a synergistic effect. Although IGF-I and cortisol implants had no significant effect after 7 days, in combination they significantly increased gill Na⁺, K⁺-ATPase activity. The results indicate that GH and cortisol can increase salinity tolerance and gill Na⁺, K⁺-ATPase activity of Atlantic salmon and together act in synergy. Although IGF-I can increase salinity tolerance in short-term treatments (2–6 days) in 12 ppt, it is less effective than GH in increasing salinity tolerance and gill Na⁺, K⁺-ATPase activity in long-term treatments (7–14 days) and in interacting with cortisol.