1. A possibility of occurrence of a hormonic principle representing presumed neurosecretory control of salt balance in teleosts was examined with the euryhaline cyprinodont Oryzias latipes with positive results.
2. Removal of the caudal aspect of the spinal cord comprising the bulk of organization of the caudal neurosecretory system or transection of the cord at the level cephalad to the site of the system in question induced serious disorder in the adaptive control of Na. No significant change resulted from the operations in total body content of K, while water moved in some measure, largely independently of total Na.
3. Injection of homogenate or crude waterly extract of the caudal portion of the spinal cord of the eel, Anguilla japonica, likewise called forth either depression or elevation of total Na level of the test Oryzias, from which the caudal portion of the spinal cord had been removed off. Such diversified effects of injection were learned to be dependent upon the history of preadaptation of the test animal; decrease of Na took place in the fish that had been kept in isotonic or hypertonic media, while increase of Na resulted in the fish that had been kept in freshwater prior to injection.
4. A view was tentatively proposed that the expected hormone of the caudal neurosecretory system would be responsible for one-way activation of certain target organ (s) which, as the gate (s) for excretion and uptake of Na, might adaptively change the mode of work.
5. Brief considerations were made concerning mainly a possibility of interaction between the caudal neurosecretory system and the widely known hypothalamo-hypophyseal system in the control of salt balance in teleosts in general.