The potential effects of growth hormone (GH), prolactin (Prl), and triiodothyronine (T₃) on gill Na⁺,K⁺-ATPase activity and corticosteroid receptor (CR) concentration (B_max) and dissociation constant (K_d) were examined in juvenile Atlantic salmon (Salmo salar). Compared to controls, fish injected with GH (ovine, 5.0 μg g⁻¹) had significantly greater gill Na⁺,K⁺-ATPase activity after 7 and 14 days. Gill CRB_maxandK_dwere significantly elevated on day 7, but not day 14. T₃also significantly increased CRB_max. The effect of GH on CRB_maxwas also additive with T₃(5.0 μg g⁻¹) treatment. There was a synergistic effect on CRB_maxwhen purified coho salmon GH (csGH, 0.1 μg g⁻¹) was injected in combination with T₃(1.6 μg g⁻¹). Prl (ovine, 5.0 μg g⁻¹; purified coho salmon, 0.1 μg g⁻¹) did not significantly alter gill CRB_max. Although Prl limited the increase in CRB_maxby GH, the effect was not signicant. T₃and Prl did not have an effect onK_d. GH significantly increased gill Na⁺,K⁺-ATPase activity, T₃administration did not have a significant effect, and Prl-treated fish had significantly lower gill Na⁺,K⁺-ATPase activity. The results indicate that T₃acts additively with GH, while Prl has no effect in regulating CRB_max. An increase in cytosolic CR by GH and T₃, but not Prl, may regulate gill responsiveness to cortisol and be an important mechanism in the endocrine control of physiological changes during the parr–smolt transformation.