Juvenile coho salmon Oncorhynchus kisutch were maintained for 3 weeks in well water supplemented with low concentrations of NaCl (12 to 24 mM) or CaCl2(6 to 12 mM) to determine the effect on plasma thyroxine (T4) concentrations. Exposure to 12 to 24 mM NaCl increased plasma T4concentrations, whereas exposure to 6 to 12 mM CaCl2had no effect. A 24-hour exposure to seawater (75% or 345 mM NaCl) generally reduced T4concentrations. These findings suggest that the beneficial effects of salt supplementation during early salmonid development may be due, in part, to stimulation of thyroid activity.