1. Factors controlling adaptive hypertrophy, which occurs when marine, or potentially marine, birds drink salt water, have been investigated in geese and ducks using changes in salt-weight weight, RNA and DNA contents as indices of this process. 2. Unilateral post-ganglionic denervation in geese prevented the changes in [RNA] and [RNA]:[DNA] that occurred in the intact gland of birds given salt water for 24 hr; denervation had no significant effect in birds on fresh water throughout. 3. Atropine treatment also prevented the adaptive changes in geese given salt water. 4. In ducks give 0.3 M-NaCl for 48 hr salt-gland weight, [RNA] and [RNA]:[DNA] increase markedly. Treatment of ducks drinking fresh water with large doses of corticosterone and mammalian ACTH for 48 hr had no significant effects on salt-gland weight, RNA or DNA; mammalian prolactin treatment for 48 hr significantly raised [RNA]. 5. No changes in the total amount of DNA in the glands were observed in these experiments, thus indicating that hyperplasia does not occur within 48 hr of a bird first drinking salt water. 6. It is concluded that adaptive hypertrophy is controlled by secretory nerves, and that hormones, if they play any part in this process, have a permissive or secondary role. It is suggested that hypertrophy and the maintenance of the secretory cells in the fully-adapted state may be obligatorily related to secretory activity induced by cholinergic secretory nerves.