Atrial natriuretic peptide (ANP) decreases plasma Na concentration and promtes seawater (SW) adaptation in eels. The hyponatremia may most probably be caused by increased branchial extrusion of Na , but the mechanism has not been determined yet. The present study examined initially the effects of ANP on branchial Na efflux in vivo using isotopic ²²Na. However, the efflux rate was not altered by infusion of a hyponatremic dose of ANP (5 pmol·kg⁻¹·min⁻¹). Therefore, we sought to examine whether the ANP-mediated hyponatremia is caused by a decrease in the uptake of Na from the environment. Since a decrease in drinking was highly correlated with a degree of hyponatremia, conscious SW eels were infused with dilute SW into the stomach at a normal drinking rate to offset the antidipsogenic effect of ANP. Under this regimen, the hyponatremic effect of ANP was abolished. Then, we examined the site of Na absorption in the alimentary tract by measuring the changes in ion composition of intraluminal fluid along the tract. Since Na was absorbed at the esophagus and anterior/middle intestine, a sac was prepared at each site and the effects of ANP were examined in situ in conscious SW eels. ANP infusion did not alter Na absorption at the esophagus, but it profoundly reduced the absorption at the intestine. Together with our previous finding that ANP does not alter renal Na excretion, we propose that ANP reduces plasma Na concentration in SW eels by inhibiting drinking and subsequent absorption of Na by the intestine.