Unidirectional fluxes of Na+ obtained in perfused preparation and mucosal enzyme equipment (alkaline phosphatase, ouabain-sensitive Na+, K+-ATPase) have been determined in the middle and posterior intestine of freshwater (FW) and sea-water (SW) adapted trout. In FW, influxes and outfluxes were higher in the middle than in the posterior intestine, although net fluxes were similar. SW adaptation induced an increase of influxes and net fluxes mainly in the posterior intestine. SW adaptation decreased the alkaline phosphatase activity only in the posterior intestine. Na+,K+-ATPase activity was always higher in the middle than in the posterior intestine in FW and SW and increased in both parts by SW adaptation. Thus, it seems that SW adaptation of rainbow trout modifies Na intestinal absorption principally in its posterior part and in relation with the Na+, K+-ATPase activity.