Changes in plasma cortisol concentration and the rate of intestinal water absorption in vitro were examined at intervals after transferring the eel from fresh water to sea water and after treating the freshwater eel with ACTH or cortisol. When eels were transferred from fresh water to sea water, plasma cortisol level increased temporarily after 2 to 4 hr. The highest concentration observed 2 hr after transfer was 9.6±1.6μg/100ml; after 24hr, the concentration returned to the initial freshwater level (3.8±0.9μg/100ml). On the other hand, the increase in the rate of water absorption in the gut was observed 12hr after transfer, and maximum increase was attained after 48 hr. Thereafter the intestine maintained a higher rate than that of the freshwater eel. Essentially similar changes in plasma cortisol and intestinal water flux were observed in the freshwater eel injected with ACTH or cortisol; maximum cortisol level was observed 1-2hr after injection, and intestinal water flux increased maximally after 24 hr, although the highest concentration of cortisol was about 20, μg/100ml, and intestinal water flux returned to the initial level 7 days after the injection. These results indicate that the increased rate of water absorption in the seawater eel is induced by the short-term increase in circulating cortisol. Furthermore, the delayed and prolonged response of the gut to circulating cortisol suggests involvement of some metabolic process for induction and maintenance of increased rate of water absorption in the eel gut in sea water.