The naturally occurring corticosteroids, cortisol and aldosterone, failed to promote survival of hypophysectomized Fundulus heteroclitus in fresh water. Extracts of Fundulus interrenal tissue, carp corpuscles of Stannius, and hog renin were ineffective. Injection of whole rat-pituitary brei was partially successful. Highly purified prolactin maintained survival, although the recipients did not eat normally. A synergic action of prolactin with some unidentified pituitary hormone is suspected.