The seawater-adapting actions of GH, which are independent of growth, were studied in juvenile rainbow trout (Salmo gairdneri). Hormones examined were chum salmon (Oncorhynchus keta) GH (sGH) and prolactin (sPRL), and ovine GH (oGH). Plasma Na levels of freshwater-adapted fish peaked 24 h after transfer to 67% seawater and remained high for at least 48 h. Twenty-four hours after transfer, plasma Na levels were inversely correlated to body weight. In order to limit size and growth effects in all subsequent experiments, fish having a narrow range of body weights, fed a fixed diet, and injected with hormones over a short time-period were used. Plasma Na levels 24 h after transfer to 80% seawater were reduced significantly by sGH (0.25 and 2.5 μg/g) and oGH (2.5 μg/g) compared with saline injections, whereas sPRL (2.5 μg/g) had no significant effect. All the GH-treated fish had lower plasma Mg levels than controls; Ca levels were significantly reduced by the high dose of sGH. Salmon prolactin had no effect on concentrations of divalent ions. When the effects of a range of doses (0.01-1.25 μg/g) of sGH on plasma ion levels was tested, 0.25 μg/g was the most potent in reducing Na and Mg levels, while 1.25 μg/g alone reduced plasma Ca concentrations significantly. These studies show that the seawater-adapting actions of GH in trout are specific to that hormone and are not consequent to an increase in size.