ABSTRACT Salmon smolts adapted to sea water maintain a high rate of turnover of both sodium and chloride, but when adapted to fresh water the rate of turnover is low. Only a small part of the influx takes place through the gut. On immediate transfer from sea water to dilute sea water or to fresh water the influxes decline rapidly, but on transfer from fresh water to sea water the restoration of the fluxes takes place slowly. The alternative hypotheses that the rapid changes are due to exchange diffusion or to rapid adjustments of the sodium pump are discussed.