With the aim of comparing the effects of oral T 3 and NaC1 administration on trout hypoosmoregulatory mechanisms, three groups of rainbow trout (Oncorhynchus mykiss Walbaum) held in freshwater (FW) were fed a basal diet (C), the same diet containing 8.83 ppm of 3,5,3'-triiodo-L-thyronine (T3) (T) or 10% (w/w) NaC1 (N) respectively for 30 d. They were then transferred to brackish water (BW) for 22 d and fed on diet C. Gill (Na++K+)-ATPase activity and its dependence on ATP, Na + and pH, number of gill chloride cells (CC), serum T 3 level as well as fish growth, condition factor (K) and mortality were evaluated. During the FW phase, as compared to C trout, T trout showed a two fold higher serum T 3 level, had unchanged gill (Na~+K+)-ATPase activity and increased CC number, whereas N trout showed higher gill (Na~+K+)- ATPase activity and CC number. At the end of the experiment the enzyme activity was in the order T > N > C groups and all groups showed similar CC number. Both treatments changed the enzyme activation kinet- ics by ATP and Na +. A transient increase in K value occurred in N group during the period of salt adminis- tration. In BW, T and N groups had higher and lower survival than C group respectively. Other parameters were unaffected by the treatments. This trial suggests that T 3 administration promotes the development of hypoosmoregulatory mechanisms of trout but it leaves the (Na++K+)-ATPase activity unaltered till the transfer to a hyperosmotic environment.
