1. Unidirectional Na+ fluxes obtained in perfused preparation of intestine and Na+ effluxes from enterocytes isolated by a specific method have been determined in the middle intestine of freshwater rainbow trout. 2. Fish were fed semi-purified diets containing 8% by wt of either cod liver oil as reference, grape seed oil or hydrogenated coconut oil which are both deficient in fatty acids (FA) of the (n-3) series essential for the trout. 3. Important modifications in the Na+ intestinal absorption were induced by these deficient diets when measured in perfused intestine. 4. Conversely, no changes were seen in the Na+ active or passive effluxed when measured in enterocytes isolated from trout fed the same diets. 5. Na+,K+-ATPase activities were also unmodified by any diet. 6. It is concluded that deprivation of essential fatty acids reduces Na+ absorption capabilities of trout intestine, an effect which is mainly located at the apical membrane of enterocytes. 7. These results strongly demonstrate that apical entry and active serosal transport are separately modified in trout intestine by EFA deficient diet. 8. Furthermore the FA of the (n-3) series are essential for the retention of functional properties of membranes and ionic permeabilities in particular.