Gene expression for growth hormone (GH)/prolactin (PRL)/somatolactin (SL) family hormones in the pituitaries of homing chum salmon were examined, because gene expression for these hormones during ocean-migrating phases remains unclear. Fish were collected in the winter Gulf of Alaska, the summer Bering Sea and along homing pathway in the Ishikari River-Ishikari Bay water system in Hokkaido, Japan in autumn. The oceanic fish included maturing adults, which had developing gonads and left the Bering Sea for the natal river by the end of summer. The absolute amounts of GH, PRL and SL mRNAs in the pituitaries of the maturing adults in the summer Bering Sea were 5- to 20-fold those in the winter Gulf of Alaska. The amount of GH mRNA in the homing adults at the coastal seawater (SW) areas was smaller than that in the Bering fish, while the amount of PRL mRNA remained at the higher level until fish arrived at the Ishikari River. The gill Na(+),K(+)-ATPase activity in the coastal SW fish and the plasma Na(+) levels in the brackish water fish at the estuary were lowered to the levels that were comparable to those in the fresh water (FW) fish. In conclusion, gene expression for GH, PRL and SL was elevated in the pituitaries of chum salmon before initiation of homing behavior from the summer Bering Sea. Gene expression for GH is thereafter lowered coincidently with malfunction of SW adaptability in the breeding season, while gene expression for PRL is maintained high until forthcoming FW adaptation. Crown Copyright (C) 2010 Published by Elsevier Inc. All rights reserved.