In the eel, a low dose of tilapia growth hormone (tGH) (45 ng/g body wt), like ovine GH (oGH), induces a decrease in plasma thyroxine and a concomitant increase in plasma triiodothyronine, which result from a stimulation of peripheral conversion of thyroxine to triiodothyronine. Salmon prolactin (sPrl), unlike ovine Prl (oPrl), has no such action. Recognition of this specific action of growth hormone (GH) on production of active thyroid hormone (T3) opens up a new approach to the problem of the action of both hormones (GH, T3) in growth and in seawater adaptation of fish.