In freshwater-acclimated rainbow trout a single intraperitoneal injection of ovine TSH significantly elevated plasma thyroxine (T4) levels within I h after the injection. In seawateradapted trout the increase in T, after TSH-treatment was not evident until 6 h after the injection. TSH caused a transient fall in plasma Na+ and C1-between 3 h and 9 h after the injection in seawater-adapted fish and plasma Na+ was lowered in freshwater-adapted trout 24 h after the injection. Although there were clear histological changes in the thyroid gland of freshwater-adapted trout after TSH-injection, no such changes were evident in seawateradapted fish.Plasma thyroid hormone levels and thyroid histology in freshwater-adapted rainbow trout and coho salmon transferred to sea water, and seawater-adapted trout transferred to fresh water showed no consistent changes compared with controls.The data are interpreted to indicate that although ambient salinity may have indirect effects on thyroid activity there is no direct involvement in ionic or osmotic regulation in the two species.
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