When Japanese cultured eels, AnguilIa japonica, were transferred from freshwater to sea-water, their body weight decreased by 15~o on the second day and then recovered gradually to the original weight within a week. Serum sodium content increased by 58~o on the second day, and thereafter decreased to a constant level. Isolated intestines from these eels were bathed on both sides, mncosal and scrosal, by identical Ringer solution. The water and sodium transport was very active from mucosa to serosa in such isolated intestines of sea-water-adapted eels. This active movement was particularly dominant during the week after transferring the eel to seawater, and a peak was reached on the fifth day. The molar ratio of water to sodium in the fluid moving across the intestinal wall was higher in sea-water-adapted eels than in freshwater eels. The isolated intestines taken from silver eels during their catadromous migration, but still in the river, showed a larger amount of water transport and a higher molar ratio of water to sodium than intestines from yellow sedentary eels. These adaptive properties are a prerequisite for the marine life of the eel.