Expression of the Na(+)-K(+)-ATPase alpha-subunit was investigated in the gill and trunk kidney of Salmo trutta. Groups of freshwater (FW) fish were treated with various hormones [cortisol: 3 x 4.0 micrograms/g; recombinant salmon growth hormone (rsGH): 3 x 0.25 micrograms/g; salmon prolactin (sPRL): 3 x 0.25 micrograms/g; recombinant bovine insulin-like growth factor-I (rbIGF-I): 2 x 0.01 micrograms/g; or 2 x 0.1 micrograms/g] or transferred to 25 parts per thousand seawater (SW) and sampled after 1, 2, 3, and 50 days. Total RNA was analyzed by Northern blotting using Xenopus laevis Na(+)-K(+)-ATPase alpha-subunit cDNA as probe. The probe detected a 3.8-kb transcript. Relative to untreated FW control fish, the abundance of alpha-subunit Na(+)-K(+)-ATPase mRNA in gill tissue increased 1.7-to 2.5-fold after treatment with cortisol, rsGH, and rbIGF-I and after transfer to SW. Na(+)-K(+)-ATPase enzyme activity was also significantly stimulated in these groups, except at 0.01 micrograms/g rbIGF-I. sPRL was without effect. In the kidney, alpha-subunit mRNA level and Na(+)-K(+)-ATPase activity were unaffected by hormone treatment and SW transfer. The results indicate that an increased abundance of alpha-subunit mRNA is part of the molecular mechanism behind the increased gill Na(+)-K(+)-ATPase activity induced by SW transfer, cortisol, GH, and IGF-I.