The plasma of rainbow trout was essentially isosmotic with the medium in 7.5‰ salinity. Osmotic concentrations of plasma were higher by 5% to 12% in an exercised fish in all salinities at both temperatures when compared to those of a resting fish. However, the chloride concentrations of exercised and resting fishes were not significantly different. Both osmotic and chloride concentrations of plasma were higher at 5 °C than those at 15 °C.