The goal of this study was to determine the effect of the changes in gill morphology induced by dietary salt feeding on several aspects of gill function in rainbow trout Oncorhynchus mykiss maintained in fresh water with specific emphasis on Cl − uptake ( J IN Cl − ) and acid–base regulation. The addition of 11% NaCl to the diet caused J IN Cl − to be reduced by c. 45% from 214·4 ± 26·7 to 117·3 ± 17·4 µmol kg −1 h −1 (mean ±...