The amount of eel plasma cortisol was determined by the method of Van der Vies (1961) with some modification. The plasma cortisol level in the intact eels kept in either freshwater or seawater was about 4μg/100ml. Single intraperitoneal injection of ACTH caused a marked increase in the level of this substance after 2 hrs. In the hypophysectomized eel kept for ten days in freshwater, the cortisol concentration decreased to 0.3μg/100ml. Transfer of the intact eel from freshwater to seawater resulted in a significant increase in its concentration within 2 hrs., whereas significant increase was not observed when the eel was retransferred from seawater to freshwater. Moreover, the increase after transfer to seawater was not observed when the eel had previously been hypophysectomized. These data suggest that interrenal of the eel is under direct control of pituitary. Possible involvement of cortisol in the adaptation mechanism to seawater in the eel was discussed.